CHAPTER V.
ON REFLECTING TELESCOPES.
SECT. 1.—HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF THE CONSTRUCTION OF THESE INSTRUMENTS.
Reflecting telescopes are those which represent the images of distant objects by reflection, chiefly from concave mirrors.
Before the achromatic telescope was invented, there were two glaring imperfections in refracting telescopes, which the astronomers of the 17th century were anxious to correct. The first was its very great length when a high power was to be applied, which rendered it very unwieldy and difficult to use. The second imperfection was the incorrectness of the image as formed by a single lens. Mathematicians had demonstrated that a pencil of rays could not be collected in a single point by a spherical lens, and also that the image transmitted by such a lens would be in some degree incurvated. After several attempts had been made to correct this imperfection by grinding lenses to the figure of one of the conic sections, Sir I. Newton happened to commence an examination of the colours formed by a prism; and having, by the means of this simple instrument, discovered the different refrangibility of the rays of light—to which we have several times adverted in the preceding descriptions—he then perceived that the errors of telescopes, arising from that cause alone, were some hundred times greater than such as were occasioned by the spherical figure of lenses; which induced this illustrious philosopher to turn his attention to the improvement of telescopes by reflection.
It is generally supposed that Mr. James Gregory—a son of the Rev. John Gregory, minister of Drumoak in the county of Aberdeen—was the first who suggested the construction of a reflecting telescope. He was a young man of uncommon genius, and an eminent mathematician; and in the year 1663, at the age of only 24, he published in London, his treatise entitled ‘Optica Promota,’ in which he explained the theory of that species of reflecting telescope which still bears his name, and which he stated as being his own invention. But as Gregory, according to his own account, was endowed with no mechanical dexterity, and could find no workman capable of realizing his invention—after some fruitless attempts to form proper specula, he was obliged to give up the pursuit; so that this telescope remained for a considerable time neglected. It was several years after Gregory suggested the construction of reflecting telescopes, till Newton directed his attention fully to the subject. In a letter addressed to the secretary of the Royal Society, dated in February, 1672, he says, ‘Finding reflections to be regular, so that the angle of reflection of all sorts of rays was equal to the angle of incidence, I understood that, by their mediation, optic instruments might be brought to any degree of perfection imaginable, providing a reflecting substance could be found which would polish as finely as glass, and reflect as much light as glass transmits, and the art of communicating to it a parabolic figure be also obtained. Amidst these thoughts I was forced from Cambridge by the intervening plague, and it was more than two years before I proceeded further.’
It was towards the end of 1668, or in the beginning of the following year, when Newton, being obliged to have recourse to reflectors, and not relying on any artificer for making the specula, set about the work himself, and early in the year 1672, completed two small reflecting telescopes. In these he ground the great speculum into a spherical concave, although he approved of the parabolic form, but found himself unable to accomplish it. These telescopes were of a construction somewhat different from what Gregory had suggested, and though only 6 inches long, were considered as equal to a 6 feet common refracting telescope. It is not a little singular, however, that we hear no more about the construction of reflectors till more than half a century afterwards. It was not till the year 1723, that any reflectors were known to have been made, adapted to celestial observations. In that year, Mr. Hadley, the inventor of the reflecting quadrant, which goes by his name, published in No. 376 of the Philosophical Transactions, an account of a large reflector on Newton’s plan, which he had just then constructed, the performance of which left no room to doubt that this invention would remain any longer in obscurity. The large speculum of this instrument was 62⅝ inches focal distance and 5 inches diameter, was furnished with magnifying powers of from 190 to 230 times, and equalled in performance the famous aerial telescope of Huygens of 123 feet in length.[25] Since this period, the reflecting telescope has been in general use among astronomers in most countries of Europe, and has received numerous improvements, under the direction of Short, Mudge, Edwards and Herschel—the last of whom constructed reflectors of 7, 10, 20, and even 40 feet in focal length, which far surpassed, in brightness and magnifying power, all the instruments of this description, which had previously been attempted.
I shall now proceed to give a brief sketch of the nature of a reflecting telescope, and the different forms in which they have been proposed to be constructed.
Fig. 62 represents the reflecting telescope as originally proposed by Gregory. ABEF represents a tube open at AF towards the object; at the other end is placed a concave speculum BE, with a hole CD in its centre, the focus of which is at e. A little beyond this focus, towards the object end of the telescope AF, is placed another small concave mirror G, having its polished face turned towards the great speculum, and is supported by an arm GH fastened to a slider connected with the tube. At the end of the great tube BE is screwed in a small tube CDKI, containing a small plano-convex lens IK. Such are the essential parts of this instrument and their relative positions. It will be recollected in our description of the properties of concave mirrors (see page 92), that, when rays proceed from a distant object, and fall upon a concave-speculum, they paint an image or representation of the object in its focus before the speculum. Now suppose two parallel rays ab falling on the speculum BE, in cd; they are reflected to its focus e where an inverted image of the object is formed. This image is formed at a little more than the focal distance of the small speculum from its surface, and serves as it were for an object on which the small mirror may act. By the action of this mirror this first image is reflected to a point about f, where a second image is formed very large and erect. This image is magnified in the proportion of fG to eG, the rays from which are transmitted to the eye glass IK, through which the eye perceives the object clear and distinct, after the proper adjustments have been made.
| figure 66. | |
| fig. 62. |  |
| fig. 63. | |
| fig. 64. | |
| fig. 65. |
Suppose the focal distance of the great mirror was 9 inches, and the focal distance of the small mirror 1½ inch—were we to remove the eye piece of this telescope, and look through the hole of the great mirror, we should see the image of the object depicted upon the face of the small speculum, and magnified, in the proportion of 9 to 1½, or, 6 times, on the same principle as a common convex object glass 9 inches focal length, with an eye glass whose focus is 1½ inch magnifies 6 times. This may be regarded as the first part of the magnifying power. If now, we suppose the small speculum placed a little more than 1½ inch from the image formed by the great speculum, a second image is formed about f, as much exceeding the first in its dimensions as it exceeds it in distance from the small speculum, on the principle on which the object glass of a compound microscope forms a large image near the eye glass. Suppose this distance to be 9 times greater, then the whole magnifying power will be compounded of 6 multiplied by 9, or 54 times. As a telescope it magnifies 6 times, and in the microscope part 9 times.—Such is a general idea of the Gregorian telescope, the minute particulars and structure of which can only be clearly perceived by a direct inspection of the instrument.
The Newtonian Reflector.—This instrument is somewhat different both in its form and in its mode of operation from that of Gregory. It is represented in fig. 63, where BAEF is the tube, and BE, the object concave mirror, which reflects the parallel rays ab to a plane speculum G, placed 45°, or half a right angle to the axis of the concave speculum. This small plane reflector must be of an oval form, the length of the oval should be to the breadth as 7 to 5, on account of the obliquity of its position. It is supported on an arm fixed to the side of the tube; an eye-glass is placed in a small tube, moveable in the larger tube, so as to be perpendicular to the axis of the large reflector, the perpendicular line passing through the centre of the small mirror. The small mirror is situated between the large mirror and its focus, that its distance from this focal point may be equal to the distance from the centre of the mirror to the focus of the eye-glass. When the rays ab from a distant object fall upon the large speculum at cd, they are reflected towards a focus at h; but being intercepted by the plane mirror G, they are reflected perpendicularly to the eye-glass at I, in the side of the tube, and the image formed near that position at e is viewed through a small plano-convex lens. The magnifying power of this telescope is in the proportion of the focal distance of the speculum to that of the eye-glass. Thus, if the focal distance of the speculum be 36 inches, and that of the eye-glass 1/3 of an inch, the magnifying power will be 108 times. It was this form of the reflecting telescope, that Newton invented, which Sir. W. Herschel adopted, and with which he made most of his observations and discoveries.
The Cassegrainian Reflector.—This mode of the reflecting telescope, suggested by M. Cassegrain, a Frenchman, is represented in fig. 64. It is constructed in the same way as the Gregorian, with the exception of a small convex speculum G being substituted in the room of the small concave in Gregory’s construction. As the focus of a convex mirror is negative, it is placed at a distance from the large speculum equal to the difference of their foci, that is, if the focal length of the large speculum be 18 inches, and that of the small convex 2 inches, they are placed at 16 inches distant from each other, on a principle similar to that of the Galilean telescope, in which the concave eye-glass is placed within the focus of the object-glass by a space equal to the focal length of the eye-glass. In this telescope, likewise, instead of two there is only one image formed, namely that in the focus of the eye-glass; and, on this account some are of opinion that the distinctness is considerably greater than in the Gregorian. Mr. Ramsden was of opinion that this construction is preferable to either of the former reflectors, because the aberrations of the two metals have a tendency to correct each other, whereas in the Gregorian both the metals being concave, any error in the specula will be doubled. It is his opinion that the aberrations in the Cassegrainian construction to that of the Gregorian is as 3 to 5. The length of this telescope is shorter than that of a Gregorian of equal focal length, by twice the focal length of the small mirror, and it shows every thing in an inverted position, and consequently is not adapted for viewing terrestrial objects.
Dr. Hook’s Reflector.—Before the reflecting telescope was much known, Dr. Hook contrived one, the form of which is represented, fig. 65, which differs in little or nothing from the Gregorian, except that the eye-glass I is placed in the hole of the great speculum BE.
Martin’s Reflector.—Mr. Bengamin Martin, a distinguished writer on optical and philosophical science, about a century ago, described a new form of the reflecting telescope, approximating to the Newtonian structure, which he contrived for his own use. It is represented in fig. 66. ABEF is the tube, in which there is an opening or aperture OP, in the upper part. Against this hole within the tube is placed a large plane speculum GH, at half a right angle with the axis or sides of the tubes, with a hole CD perforated through its middle. The parallel rays a b falling on the inclined plane GH are reflected perpendicularly and parallel on the great speculum BE in the bottom of the tube. From thence they are reflected converging to a focus e through the hole of the plane mirror CD, which being also the focus of the eye-glass IK, the eye will perceive the object magnified and distinct.
In the figures referred to in the above descriptions, only one eye-glass is represented to avoid complexity; but in most reflecting telescopes, the eye-piece consists of a combination of two plano-convex glasses, as in fig. 67, which produces a more correct and a larger field of view than a single lens. This combination is generally known by the name of the Huygenian eye-piece which shall be described in the section on the eye-pieces of telescopes.
The following rule has been given for finding the magnifying power of the Gregorian telescope:—Multiply the focal distance of the great mirror by the distance of the small mirror from the image next the eye; and multiply the focal distance of the small mirror by the focal distance of the eye-glass; then divide the product of the former multiplication by the product of the latter, and the quotient will express the magnifying power. The following are the dimensions of one of the reflecting telescopes constructed by Mr. Short—who was long distinguished as the most eminent maker of such instruments, on a large scale, and whose large reflectors are still to be found in various observatories throughout Europe.
The focal distance of the great mirror 9.6 inches; or P m, fig. 67, its breadth FD 2.3; the focal distance of the small mirror L n 1.5—or 1½ inch—its breadth g h 0.6—or 6/10 of an inch; the breadth of the hole in the great mirror UV, 0.5—or half an inch—the distance between the small mirror and the next eye-glass LR, 14.2; the distance between the two eye-glasses SR, 2.4; the focal distance of the eye-glass next the metal, 3.8.; and the focal distance of the eye-glass next the eye, S a 1.1, or one inch and one tenth. The magnifying power of this telescope was about 60 times. Taking this telescope as a standard, the following table of the dimensions and magnifying powers of Gregorian reflecting telescopes, as constructed by Mr. Short, has been computed.
figure 67.
INDEX:
A: Focal distance of the great mirror.
B: Breadth of the great mirror.
C: Focus of the small speculum.
D: Breadth of the hole in the great speculum.
E: Distance between the small speculum and the first eye-glass.
F: Focal distance of the glass next the metals.
G: Focal distance of the glass next the eye.
H: Distance between the plain sides of the two glasses.
I: Magnifying power.
J: Distance between the second glass and the small eye-hole.
| A. | B. | C. | D. | E. | |||||
|---|---|---|---|---|---|---|---|---|---|
| P | m | D | F | L | n | U | V | L | R |
| In. | Dec. | In. | Dec. | In. | Dec. | In. | Dec. | In. | Dec. |
| 5. | 65 | 1. | 54 | 1. | 10 | 0. | 31 | 8. | 54 |
| 9. | 60 | 2. | 30 | 1. | 50 | 0. | 39 | 14. | 61 |
| 15. | 50 | 3. | 30 | 2. | 14 | 0. | 50 | 23. | 81 |
| 36. | 00 | 6. | 26 | 3. | 43 | 0. | 65 | 41. | 16 |
| 60. | 00 | 9. | 21 | 5. | 00 | 0. | 85 | 68. | 17 |
| F. | G. | H. | I. | J. | ||||
|---|---|---|---|---|---|---|---|---|
| R | S | R | S | |||||
| In. | Dec. | In. | Dec. | In. | Dec. | In. | ||
| 2. | 44 | 0. | 81 | 1. | 68 | 39 | 0. | 41 |
| 3. | 13 | 1. | 04 | 2. | 09 | 39 | 0. | 53 |
| 3. | 94 | 1. | 31 | 2. | 63 | 86 | 0. | 66 |
| 5. | 12 | 1. | 71 | 3. | 41 | 165 | 0. | 85 |
| 6. | 43 | 2. | 14 | 4. | 28 | 243 | 1. | 07 |
Mr. Short—who was born in Edinburgh in 1710, and died near London, 1768—was considered as the most accurate constructor of reflecting telescopes, during the period which intervened from 1732, to 1768. In 1743, he constructed a reflector for Lord Thomas Spencer, of 12 feet focal length, for which he received 600 guineas. He made several other telescopes of the same focal distance, with greater improvements and higher magnifiers; and in 1752, finished one for the king of Spain, for which, with its whole apparatus, he received £1200. This was considered the noblest instrument of its kind that had then been constructed, and perhaps it was never surpassed, till Herschel constructed his twenty and forty feet reflectors. High as the prices of large telescopes now are, Mr. Short charged for his instruments at a much higher rate than opticians now do, although the price of labour, and every other article required in the construction of a telescope, is now much dearer. But he had then scarcely any competitor, and he spared neither trouble nor expense to make his telescopes perfect, and put such a price upon them as properly repaid him. The following table contains a statement of the apertures, powers, and prices of Gregorian telescopes, as constructed by Mr. James Short.[26]
INDEX:
A: Number.
B: Focal length in inches.
C: Diameter of aperture in inches.
D: Prices in guineas.
| A. | B. | C. | Magnifying powers. | D. | |
|---|---|---|---|---|---|
| 1 | 3 | 1.1 | 1 Power of | 18 times | 3 |
| 2 | 4½ | 1.3 | 1 Power of | 25 times | 4 |
| 3 | 7 | 1.9 | 1 Power of | 40 times | 6 |
| 4 | 9½ | 2.5 | 2 Powers | 40 and 60 times | 8 |
| 5} | 12 | 3.0 | 2 Powers | 55 and 85 times | 10 |
| 6} | 12 | 3.0 | 4 Powers | 35, 55, 85, and 110 times | 14 |
| 7 | 18 | 3.8 | 4 Powers | 55, 95, 130, and 200 times | 20 |
| 8 | 24 | 4.5 | 4 Powers | 90, 150, 230, and 300 times | 35 |
| 9 | 36 | 6.3 | 4 Powers | 100, 200, 300, and 400 times | 75 |
| 10 | 48 | 7.6 | 4 Powers | 120, 260, 380, and 500 times | 100 |
| 11 | 72 | 12.2 | 4 Powers | 200, 400, 600, and 800 times | 300 |
| 12 | 144 | 18.0 | 4 Powers | 300, 600, 900, and 1200 times | 800 |
From this table, it appears that Mr. Short charged 75 guineas for a 3 feet reflector, whereas such an instrument is now marked in the London opticians’ catalogues at £23, when mounted on a common brass stand, and £39. 18s., when accompanied with rack-work motions and other apparatus. It is now generally understood that in the above table, Short always greatly overrated the higher powers of his telescopes. By experiment they were generally found to magnify much less than here expressed.
General remarks on Gregorian Reflectors.—1. In regard to the hole UV, of the great speculum—its diameter should be equal, or nearly so, to that of the small speculum L, fig. 67. For if it be less, no more parallel rays will be reflected than if it were equal to g h, and it may do harm in contracting the visible area within too narrow limits. Nor must it be larger than the mirror L, because some parallel rays will then be lost, and those of most consequence as being nearest the centre. 2. The small hole at e to which the eye is applied, must be nicely adjusted to the size of the cone of rays proceeding from the nearest lens S. If it be larger, it will permit the foreign light of the sky or other objects to enter the eye, so as to prevent distinct vision; for the eye should receive no light, but what comes from the surface of the small mirror L. If the hole be smaller than the cylinder of rays at e then some of the necessary light will be excluded, and the object rendered more obscure. The diameter of this hole may be found by dividing the aperture of the telescope in inches by its magnifying power. Thus, if we divide the diameter of one of Short’s telescopes, the diameter of whose large speculum is 2.30, by 60, the magnifying power, the quotient will be .0383, which is nearly the 1/25 of an inch. Sometimes this hole is made so small as the 1/50 of an inch. When this hole is, by any derangement, shifted from its proper position, it sometimes requires great nicety to adjust it, and, before it is accurately adjusted, the telescope is unfit for accurate observation. 3. It is usual to fix a plate with a hole in it, at a b, the focus of the eye glass S, of such a diameter as will circumscribe the image, so as to exhibit only that part of it which appears distinct, and to exclude the superfluous rays. 4. There is an adjusting screw on the outside of the great tube, connected with the small speculum, by which that speculum may be pushed backwards or forwards to adjust the instrument to distinct vision. The hand is applied for this purpose at T.
Newtonian Telescopes.—These telescopes are now more frequently used for celestial observations than during the last century, when Gregorian reflectors were generally preferred. Sir W. Herschel was chiefly instrumental in introducing this form of the reflecting telescope to the more particular attention of astronomers, by the splendour and extent of the discoveries which it enabled him to make. In this telescope there is no hole required in the middle of the great speculum, as in the Gregorian construction, which circumstance secures the use of all the rays which flow from the central parts of the mirror.
The following table contains a statement of the apertures and magnifying powers of Newtonian Telescopes, and the focal distances of their eye-glasses. The first column contains the focal length of the great speculum in feet; the second, its linear aperture in inches; the third, the focal distance of the single glass in decimals, or in 1000ths of an inch, and the fourth column, contains the magnifying power. This portion of the table was constructed by using the dimensions of Mr. Hadley’s Newtonian Telescope, formerly referred to, as a standard—the focal distance of the great mirror being 62½ inches, its medium aperture 5 inches, and power 208. The fifth, sixth, and seventh columns contains the apertures of the concave speculum, the focal lengths of the eye-glasses and the magnifying powers, as calculated by Sir D. Brewster, from a telescope of Mr. Hauksbee, taken as a standard; whose focal length was 3 feet 3 inches, its aperture about 4 inches, and magnifying power 226 times.
INDEX:
A: Focal distance of concave metal.
B: Aperture of concave metal.
C: Focal distance of single eye-glass.
D: Magnifying power.
E: Aperture of the concave speculum.
F: Focal length of the eye-glass.
G: Magnifying power.
| Sir D. Brewster’s Numbers. | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| A. | B. | C. | D. | E. | F. | G. | ||||
| Feet. | Inch. | Dec. | In. | Dec. | Inch. | Dec. | In. | Dec. | ||
| 0½ | 0. | 86 | 0. | 167 | 36 | 1. | 34 | 0. | 107 | 56 |
| 1 | 1. | 44 | 0. | 199 | 60 | 2. | 23 | 0. | 129 | 93 |
| 2 | 2. | 45 | 0. | 236 | 102 | 3. | 79 | 0. | 152 | 158 |
| 3 | 3. | 31 | 0. | 261 | 138 | 5. | 14 | 0. | 168 | 214 |
| 4 | 4. | 10 | 0. | 281 | 171 | 6. | 36 | 0. | 181 | 265 |
| 5 | 4. | 85 | 0. | 297 | 202 | 7. | 51 | 0. | 192 | 313 |
| 6 | 5. | 57 | 0. | 311 | 232 | 8. | 64 | 0. | 200=1/5 | 360 |
| 7 | 6. | 24 | 0. | 323 | 260 | 9. | 67 | 0. | 209 | 403 |
| 8 | 6. | 89 | 0. | 334 | 287 | 10. | 44 | 0. | 218 | 445 |
| 9 | 7. | 54 | 0. | 344 | 314 | 11. | 69 | 0. | 222 | 487 |
| 10 | 8. | 16 | 0. | 353 | 340 | 12. | 65 | 0. | 228 | 527 |
| 11 | 8. | 76 | 0. | 362 | 365 | 13. | 58 | 0. | 233 | 566 |
| 12 | 9. | 36 | 0. | 367 | 390 | 14. | 50 | 0. | 238 | 604 |
| 13 | 9. | 94 | 0. | 377 | 414 | 15. | 41 | 0. | 243 | 642 |
| 14 | 10. | 49 | 0. | 384 | 437 | 16. | 25 | 0. | 248 | 677 |
| 15 | 11. | 04 | 0. | 391 | 460 | 17. | 11 | 0. | 252 | 713 |
| 16 | 11. | 59 | 0. | 397 | 483 | 17. | 98 | 0. | 256 | 749 |
| 17 | 12. | 14 | 0. | 403 | 506 | 18. | 82 | 0. | 260 | 784 |
| 18 | 12. | 67 | 0. | 409 | 528 | 19. | 63 | 0. | 264 | 818 |
| 19 | 13. | 20 | 0. | 414 | 550 | 20. | 45 | 0. | 268 | 852 |
| 20 | 13. | 71 | 0. | 420 | 571 | 21. | 24 | 0. | 271 | 885 |
One great advantage of reflecting telescopes above common refractors, is, that they will admit of eye glasses of a much shorter focal distance, and consequently, will magnify so much the more, for the rays are not coloured by reflection from a concave mirror, if it be ground to a true figure, as they are by passing through a convex glass though figured and polished with the utmost exactness. It will be perceived from the above table, that the focal length of the eye glasses is very small, the lowest there stated being only about 1/10 of an inch, and the highest little more than ¼ of an inch focal distance. Sir W. Herschel obtained the high powers which he sometimes put upon his telescopes, by using small double convex lenses for eye glasses, some of which did not exceed the one fiftieth of an inch in focal length. When the focal length of the concave speculum, and that of the eye glass are given, the magnifying power is found by dividing the former by the latter, after having reduced the focal length of the concave speculum to inches. Thus the 6 feet speculum, multiplied by 12, produces 72 inches, which, divided by Brewster’s number for the focus of the eye glass = 200, or 1/5 of an inch, produces a quotient of 360 as the magnifying power. It has been calculated that, if the metals of a Newtonian telescope be worked as exquisitely as those in Sir W. Herschel’s 7 feet reflector, the highest power that such a telescope should bear with perfect distinctness, will be found by multiplying the diameter of the great speculum in inches, by 74, and the focal distance of the single eye glass may be found by dividing the focal distance of the great mirror by the magnifying power. Thus 6.25—the aperture in inches of Herschel’s 7 feet Newtonian—multiplied by 74 is 462½, the magnifying power; and 7 multiplied by 12, and divided by 462.5 is 0.182 of an inch, the focal distance of the single eye glass required. But it is seldom that more than one half of this power can be applied with effect to any of the planetary bodies. For general purposes the power produced by multiplying the diameter of the speculum by 30, or 40, will be found most satisfactory.
The following are the general prices of reflecting telescopes as made by the London opticians.
| £ | s. | |
|---|---|---|
| A four feet, seven inch aperture, Gregorian Reflector; with the vertical motions upon a new invented principle, as well as apparatus to render the tube more steady in observation; according to the additional apparatus of small speculums, eye-pieces, micrometers, &c. from | 80 to 120 | 0 |
| Three feet long, mounted on a plain brass stand | 23 | 2 |
| Ditto, with rack-work motions, improved mounting, and metals | 39 | 18 |
| Two feet long without rack-work, and with 4 magnifying powers, improved | 15 | 15 |
| Ditto with rack-work motion | 22 | 1 |
| Eighteen inch on a plain stand | 9 | 9 |
| Twelve inch Ditto | 6 | 6 |
The above are the prices stated in Messrs. W. and S. Joneses catalogue.
The following list of prices of the various kinds of reflecting telescopes is from Messrs. Tulley’s (of Islington) catalogue.
| £ | s. | |
|---|---|---|
| 1 foot Gregorian reflector, on pillar and claw stand, metal 2½ inches diameter, packed in a mahogany box | 6 | 6 |
| 1½ foot ditto, on pillar and claw stand, metal 3 inches diameter, packed in mahogany box | 11 | 11 |
| 2 feet ditto, metal 4 inches diameter | 16 | 16 |
| Ditto, ditto, with rack-work motions | 25 | 4 |
| 3 feet ditto, metal 5 inches diameter, with rack-work motions | 42 | 0 |
| Ditto, metal 6 inches diameter, on a tripod stand, with centre of gravity motion | 68 | 5 |
| 4 feet ditto, metal 7 inches diameter, as above | 105 | 0 |
| 6 feet ditto, metal 9 inches diameter, on an improved iron stand | 210 | 0 |
| 7 feet Newtonian reflectors, 6 inches aperture, mounted on a new and improved stand | 105 | 0 |
| Ditto, ditto, metal 7 inches diameter | 126 | 0 |
| 9 feet ditto, metal 9 inches diameter | 210 | 0 |
| 10 feet ditto, metal 10 inches diameter | 315 | 0 |
| 12 feet ditto, metal 12 inches diameter | 525 | 0 |
Comparative brightness of achromatic and reflecting telescopes. The late astronomer royal, Dr. Maskelyne, from a comparison of a variety of telescopes, was led to the following conclusion,—‘that the aperture of a common reflecting telescope, in order to show objects as bright as the achromatic must be to that of an achromatic telescope as 8 to 5,’—in other words, an achromatic whose object glass is 5 inches diameter, will show objects with as great a degree of brightness as a reflector whose large speculum is 8 inches in diameter. This result, if correct, must be owing to the small number of rays reflected from a speculum compared with the number transmitted through an achromatic object glass.
SECT. 2.—THE HERSCHELIAN TELESCOPE.
Soon after Sir William Herschel commenced his astronomical career, he introduced a new era in the history of reflecting telescopes. After he had cast and polished an immense variety of specula for telescopes of different sizes-he, at length, in the year 1782, finished a 20 feet reflector with a large aperture. Being sensible of the vast quantity of light which is lost by a second reflection from the small speculum, he determined to throw it aside altogether, and mounted this 20 feet reflector on a stand that admitted of being used without a small speculum in making front observations—that is, in sitting with his back to the object, and looking directly towards the surface of the speculum. Many of his discoveries and measurements of double stars were made with this instrument, till, at length, in the year 1785 he put the finishing hand to that gigantic speculum, which soon became the object of universal astonishment, and which was intended for his forty feet reflecting telescope; he had succeeded so well in constructing reflecting telescopes of comparatively small aperture, that they would bear higher magnifying powers than had ever previously been applied; but he found that a deficiency of light could only be remedied by an increased diameter of the large speculum, which therefore was his main object, when he undertook to accomplish a work which to a man less enterprising, would have appeared impracticable. The difficulties he had to overcome were numerous; particularly in the operative department of preparing, melting, annealing, grinding, and polishing a mass of metal that was too unwieldly to be moved without the aid of mechanical powers. At length, however, all difficulties having been overcome, this magnificent instrument was completed with all its complicated apparatus, and erected for observation, on the 28th of August, 1789, and on the same day the sixth satellite of Saturn was detected, as a prelude of still farther discoveries which were afterwards made by this instrument, in the celestial regions.
It would be too tedious to attempt a description of all the machinery and apparatus connected with this noble instrument. The reader who wishes to peruse a minute description of the stairs, ladders, platform, rollers, and of every circumstance relating to joiner’s work, carpenter’s work, smith’s work, and other particulars connected with the formation and erection of this telescope, will find the details recorded in the 85th volume of the Philosophical Transactions of the Royal Society of London, for 1795, in which there are sixty-three pages of letter press, and eighteen plates illustrative of the subject. I shall content myself with giving a short outline of the essential parts belonging to this instrument.
The tube of this telescope is made of rolled or sheet iron, joined together without rivets; the thickness of the sheets is somewhat less than 1/36 part of an inch, or 14 pounds weight for a square foot; great care was taken that the cylindrical form should be secured, and the whole was coated over three or four times with paint, inside and outside, to secure it against the damp. This tube was removed from the place in which it was formed by twenty-four men, divided into six sets; so that two men on each side, with a pole of 5 feet long in their hands, to which was affixed a piece of course cloth, 7 feet long going under the tube, and joined to a pole 5 feet long, in the hands of two other men, assisted in carrying the tube. The length of this tube is 39 feet 4 inches, the diameter 4 feet 10 inches; and, on a moderate computation, it was ascertained, that a wooden tube of proper dimensions would have exceeded an iron one in weight by at least 3000 pounds. Reckoning the circumference of the tube 15 feet, its length 39⅓ feet, and 14 lib. for the weight of a square foot, it must have contained 590 square feet, and weighed 8,260 pounds. Various hoops were fixed within the tube, and longitudinal bars of iron connecting some of them are attached to the two ends of the tube, by way of bracing the sheets, and preserving the shape perfect, when the pulleys are applied to give the necessary elevation at the upper end, and that the speculum may be kept secure at the lower end. The lower end of the tube is firmly supported on rollers that are capable of being moved forwards or backwards by a double rack, connected with a set of wheels and pinions. By an adjustment at the lower extremity of the tube, the speculum is turned to a small inclination, so that the line of collimation may not be coincident with the longitudinal axis of the tube, but may cross the tube diagonally, and meet the eye in the air at about two inches from the edge of the tube, which is the peculiarity of the construction, that supersedes the necessity of applying a second reflector. Hence no part of the head of the observer intercepts the incident rays, and the observation is taken with the face looking at the speculum, the back being turned to the object to be observed.
The large speculum is enclosed in a strong iron ring, braced across with bars of iron, and an enclosure of iron and ten sheets makes a case for it. It is lifted by three handles of iron attached to the sides of the ring, and is put into and taken out of its proper place in the tube by the help of a moveable crane, running on a carriage, which operation requires great care. The speculum is made of a metallic composition, and is 49½ inches in diameter; but the concave polished surface is only 48 inches, or 4 feet in diameter. Its thickness is 31 inches; and when it came from the cast its weight was 2118 pounds. The metals for its formation were procured at a warehouse in Thames Street, London, where they kept ingots of two kinds ready made, one of white, and the other of bell-metal; and it was composed of two ingots of bell-metal for one of white. It was not to be expected that a speculum of such large dimensions, could have a perfect figure imparted to its surface, nor that the curve, whatever it might be, would remain identically the same in changes of temperature; therefore we are not surprised when we are told, that the magnifying powers used with this telescope seldom exceeded 200; the quantity of light collected by so large a surface being the principal aim of the maker. The raising of the balcony, on which the observer stands, and the sliding of the lower end of the tube, in which the speculum rests, are effected by separate tackles, and require only occasional motions; but the elevation of the telescope requires the main tackle to be employed, and the motion usually given in altitude at once was two degrees; the breadth of the zone in which the observations were made, as the motion of the sphere in right ascension brought the objects into view. A star, however, could be followed for about a quarter of an hour. Three persons were employed in using this telescope, one to work the tackle, another to observe, and a third to mark down the observations. The elevation was pointed out by a small quadrant fixed to the main tube, near the lower end, but the polar distance was indicated by a piece of machinery, worked by a string, which continually indicated the degree and minute on a dial in the small house adjoining, while the time was shown by a clock in the same place, Miss Herschel performing the office of Registrar.
At the upper end the tube is open, and directed to the part of the heavens intended for observation, and the observer, standing on the foot board, looks down the tube, and perceives the object by rays reflected from the speculum, through the eye glass at the opening of the tube. When the telescope is directed to any objects near the zenith, the observer is necessarily at an elevation at least 40 feet from the ground. Near the place of the eye glass is the end of a tin pipe, into which a mouth-piece may be placed, so that, during an observation, a person may direct his voice into this pipe, while his eye is at the glass. This pipe, which is 1½ inch in diameter runs down to the bottom of the tube, where it goes into a turning joint, thence into a drawing tube, and out of this into another turning joint, from whence it proceeds, by a set of sliding tubes towards the front of the foundation timber. Its use is to convey the voice of the observer to his assistants, for at the last place, it divides itself into two branches, one going into the observatory, the other into the workman’s room, ascending in both places through the floor, and terminates in the usual shape of speaking trumpets. Though the voice passes in this manner through a tube, with many inflections, and through not less than 115 feet, it requires very little exertion to be well understood.
To direct so unwieldy a body to any part of the heavens at pleasure, many mechanical contrivances were evidently necessary. The whole apparatus rests upon rollers, and care was previously taken of the foundation in the ground. This consists of concentrical brick walls, the outermost 42 feet, the innermost 21 feet in diameter, 2 feet 6 inches deep under ground, 2 feet 3 inches broad at the bottom, and 1 foot 2 inches at the top, capped with paving stones 3 inches thick, and 12¾ inches broad.
In the centre is a large post of oak, framed together with braces under ground, and walled fast to brick-work to make it steady. Round this centre the whole frame is moved horizontally by means of 20 rollers, 12 upon the outer, and 8 upon the inner wall. The vertical motion is given to the instrument by means of ropes and pullies, passing over the main beam supported by the ladders. These ladders are 49 feet long, and there is a moveable gallery with 24 rollers to ease its motion. There is a stair-case intended for persons who wish to ascend into the gallery, without being obliged to go up the ladder. The ease with which the horizontal and vertical motions may be communicated to the tube may be conceived, from a remark of Sir W. Herschel, that, in the year 1789, he several times observed Saturn, two or three hours before and after its meridian passage with one single person to continue, at his directions, the necessary horizontal and vertical motions.
By this telescope the sixth and seventh satellites of Saturn were discovered, only one of which is within the reach of the 20 feet reflector, or even of a 25 feet instrument. The discovery of the satellites of the planet Uranus, however, was made by the 20 feet reflector, but only after it had been converted from the Newtonian to the Herschelian construction—which affords a proof of the superiority of the latter construction over the former when the same speculum is used. Never had the heavens before been observed with so extraordinary an instrument as the forty feet reflector. The nebulosities which are found among the fixed stars, in various regions of the heavens, appeared almost all to resolve themselves into an innumerable multitude of stars; others, hitherto imperceptible, seemed to have acquired a distinct light. On the entrance of Sirius into the field of the telescope, the eye was so violently affected, that stars of less magnitude could not immediately after be perceived; and it was necessary to wait for 20 minutes before these stars could be observed. The ring of Saturn had always before ceased to be visible when its plane was directed towards the earth; but the feeble light which it reflects in that position was enough for Herschel’s instrument, and the ring, even then, still remained visible to him.
It has been generally considered that this telescope was capable of carrying a power of 6000 times; and perhaps for the purpose of an experiment, and for trying its effect on certain objects, such a power may have been applied,—in which case the eye-glass must have been only 2/25 of an inch focal distance, or somewhat less than one twelfth of an inch. But such a power could not be generally applied, with any good effect, to the planetary bodies; and I question much whether any power above 1000 times was ever generally used. For, it is the quantity of light which the telescope collects, more than the magnifying power, that enables us to penetrate, with effect, into the distant spaces of the firmament: and hence, as above stated, the power seldom exceeded 200, which on account of the large diameter of the speculum, would enable the instrument to penetrate into the distant celestial spaces perhaps further than if a power of as many thousands of times had been applied.
Sir John Herschel, who inherits all the science, skill, and industry of his father, some time ago ground and polished a new speculum for the 20 feet tube, formerly noticed, which is connected with a stand, pulleys and other appendages, similar to those above described, though of smaller dimensions. This telescope shows the double stars exceedingly well defined, and was one of the principal instruments used in forming his catalogue of these objects which was presented to the Royal Society, in conjunction with that of Sir James South, about the year 1828. I suppose, it is likewise the same telescope with which Sir John lately made his Sidereal observations at the Cape of Good Hope.
SECT. 3.—RAMAGE’S LARGE REFLECTING TELESCOPE.
The largest front view reflecting telescope in this country—next to Herschel’s 40 feet instrument—is that which was erected at the Royal Observatory at Greenwich, in the year 1820, by Mr. Ramage of Aberdeen. The diameter of the concave reflector is 15 inches, and its focal length 25 feet. It is erected on machinery which bears a certain resemblance to that of Herschel’s, which we have now described; but the mechanical arrangements are greatly simplified, so that the instrument is manageable by an observer without an assistant. The tube is composed of a twelve-sided prism of deal 5/8 inch thick. At the mouth is a double cylinder of different diameters on the same axis; around this a cord is wound by a winch, and passes up from the small cylinder, over a pulley, and down through another pulley on to the large cylinder. When the winch, therefore, is turned to raise the telescope, the endless cord is unwound from the smaller cylinder, and wound on to the larger, the difference of the size of the two cylinders will be double the quantity raised, and a mechanical force to any extent may thus be obtained, by duly proportioning the diameters of the two cylinders: by this contrivance the necessity of an assistant is superseded. The view through this instrument first astonished those observers who had not been accustomed to examine a heavenly body with a telescope possessing so much light; and its performance was deemed quite extraordinary. But when the first impression had subsided, and different trials had been made in different states of the atmosphere, it was discovered that the central portion of the speculum was more perfectly figured than the ring bordering on the extreme edges. When the aperture was limited to ten or twelve inches, the performance as to the distinctness in its defining power, was greatly improved, and the light was so brilliant, that the Astronomer Royal was disposed to entertain an opinion, that it might equal that of a good achromatic refractor of the same dimensions. When, however, very small and obscure objects are to be observed, the whole light of the entire aperture may be used with advantage on favourable evenings.
The eye-pieces adapted to this telescope have powers which magnify the object linearly from 100 to 1500 times, which are competent to fulfil all the purposes of vision when cleared of aberration. When the telescope is placed in the plane of the meridian and elevated together with the gallery, into any required altitude, the meridional sweeps, formerly practised by Sir W. Herschel, and continued by Sir John with great success, in the examination of double stars and nebula, may be managed with great ease.
Mr. Ramage had a telescope of about the same size, erected in an open space in Aberdeen, which I had an opportunity of inspecting when I paid a visit to that gentleman in 1833; but cloudy weather prevented my obtaining a view of any celestial bodies through it. He showed me at that time two or three large speculums, from 12 to 18 inches in diameter, which he had finished some time before, and which appeared most beautifully polished. He told me, too, that he had ground and polished them simply with his hand, without the aid of any machinery or mechanical power—a circumstance which, he said, astonished the opticians of London, when it was stated, and which they considered as almost incredible. His experience in casting and polishing metals of various sizes, during a period of 15 or 16 years, qualified him to prepare specula of great lustre, and with an unusually high polish. It has been asserted that a fifty feet telescope by Ramage of 21 inches aperture was intended to be substituted for the 25 feet instrument erected at Greenwich, and the speculum it is understood, was prepared, and ready for use, provided the Navy Board was disposed to defray the expense of carrying the plan into execution. But, unfortunately, this ingenious artist was unexpectedly cut off in the midst of his career, about the year 1835.
SECT. 4.—THE AERIAL REFLECTOR—CONSTRUCTED BY THE AUTHOR.
A particular description of this telescope was given in the ‘Edinburgh New Philosophical Journal’ for April—July, 1826, conducted by Professor Jameson, the greater part of which was copied in the ‘London Encyclopedia,’ ] under the article Telescope. From this description I shall endeavour to condense a brief account of this instrument with a few additional remarks.
About the year 1822, an old speculum 27 inches in focal length, very imperfectly polished happened accidentally to come into my possession; and feeling no inclination to fit it up in the Gregorian form, I formed the resolution of throwing aside the small speculum, and attempting the front view notwithstanding the uniform assertion of opticians, that such an attempt in instruments of a small size is impracticable. I had some ground for expecting success in this attempt, from several experiments I had previously made, particularly from some modifications I had made in the construction of astronomical eye-pieces, which have a tendency to correct the aberration of the rays of light, when they proceed somewhat obliquely from a lens or speculum. In the first instance, I placed the speculum at the one end of a tube of the form of a segment of a cone—the end next the eye being somewhat wider than that at which the speculum was fixed, and its length about an inch shorter than the focal distance of the mirror. A small tube for receiving the different eye-pieces was fixed in the inside of the large tube at the end next the eye, and connected with an apparatus by which it could occasionally be moved either in a vertical or horizontal direction. With the instrument fitted up in this manner, I obtained some interesting views of the moon, and of terrestrial objects. But finding that one side of the tube intercepted a considerable portion of light from the object, I determined to throw aside the tube altogether, and to fit up the instrument on a different plan.
A short mahogany tube, about 3 inches long, was prepared, to serve as a socket for holding the speculum. To the side of this tube an arm was attached, about the length of the focal distance of the mirror, at the extremity of which a brass tube for receiving the eye-pieces, was fixed, connected with screws and sockets, by which it might be raised or depressed, and turned to the right hand or to the left, and with adjusting apparatus by which it might be brought nearer to or farther from the speculum. Fig. 69 exhibits a general representation of the instrument in profile. AB is the short tube which holds the speculum; CD the arm which carries the eye-tubes, which consists of two distinct pieces of mahogany; the part D being capable of sliding along the under side of C, through the brass sockets EF. To the under part of the socket F is attached a brass nut with a female screw, in which the male screw ab acts by applying the hand to the knob c, which serves for adjusting the instrument to distinct vision. G is the brass tube which receives the eye-pieces. It is supported by a strong brass wire de, which passes through a nut connected with another strong wire, which passes through the arm D. By means of the nut f this tube may be elevated or depressed, and firmly fixed in its proper position; and by the nut d it may be brought nearer to or further from, the arm D.
figure 69.
By the same apparatus, it is also rendered capable of being moved either in a vertical or horizontal direction: but when it is once adjusted to its proper position, it must be firmly fixed, and requires no further attention. The eye-piece represented in this figure is the one used for terrestrial objects, which consists of the tubes belonging to a pocket achromatic telescope. When an astronomical eye-piece is used, the length of the instrument extends only to the point I. In looking through this telescope, the right eye is applied at the point H, and the observer’s head is understood to be uncovered, or, at least, tightly covered with a thin cap. For those who use only the left eye, the arm would require to be placed on the opposite side of the tube, or the arm, along with the tube, be made to turn round 180 degrees.
figure 70.
Fig. 70 represents a front, or rather an oblique view of the instrument, in which the position of the speculum may be seen. All the specula which I fitted up in this form, having been originally intended for Gregorian reflectors, have holes in their centres. The eye-piece is therefore directed to a point nearly equi-distant from the hole to the left hand edge of the speculum, that is, to the point a. In one of these instruments fitted up with a four feet speculum, the line of vision is directed to the point b on the opposite side of the speculum, but, in this case, the eye-tube is removed farther from the arm, than in the former case. The hole in the centre of the speculum is obviously a defect in this construction of a reflecting telescope, as it prevents us from obtaining the full advantage of the rays which fall near the centre of the mirror; yet the performance of the instruments, even with this disadvantage, is superior to what we should previously have been led to expect.
The principal nicety in the construction of this instrument, consists in the adjustment and proper direction of the eye-tube. There is only one position in which vision will be perfectly distinct. It must be neither too high nor too low,—it must be fixed at a certain distance from the arm,—and must be directed to a certain point of the speculum. This position must be ultimately determined by experiment, when viewing terrestrial objects. A person unacquainted with this construction of the telescope, would, perhaps, find it difficult, in the first instance, to make this adjustment; but were it at any time deranged, through accident or otherwise, I can easily make the adjustment anew, in the course of a minute or two.
In pointing this telescope to the object intended to be viewed, the eye is applied at K, fig, 69, and looking along the arm, towards the eye-piece, till it nearly coincide with the object, it will, in most cases, be readily found. In this way I can easily point this instrument to Jupiter or Saturn, or to any of the other planets, visible to the naked eye, even when a power of 160 or 170 times is applied. When high magnifying powers, however, are used, it may be expedient to fix, on the upper part of the short tube in which the speculum rests, a Finder, such as that which is used in Newtonian telescopes. When the moon is the object intended to be viewed, she may be instantly found by moving the instrument till her reflected image be seen from the eye-end of the telescope on the face of the mirror.
I have fitted up several instruments of the above description with specula of 16, 27, 35, and 49 inches focal distance. One of these having a speculum of 27 inches focal length, and an astronomical eye-piece, producing a magnifying power of about 90 times, serves as a good astronomical telescope. By this instrument the belts and satellites of Jupiter, the ring of Saturn, and the mountains and cavities of the moon, may be contemplated with great ease and distinctness. With a magnifying power of 35 or 40 times, terrestrial objects appear remarkably bright and well-defined. When compared with a Gregorian, the quantity of light upon the object appears nearly doubled, and the image is equally distinct—although the speculum has several blemishes, and its surface is but imperfectly polished. It represents objects in their natural colours, without that dingy and yellowish tinge which appears when looking through a Gregorian. Another of these instruments is about four feet long. The speculum which belongs to it is a very old one: when it came into my possession, it was so completely tarnished, as scarcely to reflect a ray of light. After it was cleaned, it appeared to be scarcely half polished, and its surface is covered with yellowish stains which cannot be erased. Were it fitted up upon the Gregorian plan, it would, I presume, be of very little use, unless when a very small magnifying power was applied. Yet, in its present form, it bears, with distinctness, a magnifying power of 130 times, and is equal in its performance to a 3½ feet achromatic. It exhibits distinct and interesting views of the diversities of shade, and of the mountains, vales, cavities, and other inequalities of the moon’s surface. With a power of about 50 times, and a terrestrial eye-piece, it forms an excellent telescope for land objects, and exhibits them in a brilliant and novel aspect. The smallest instrument I have attempted to construct on this plan, is only 5½ inches focal distance, and 1¾ inch diameter. With a magnifying power of about 15 times, it shows terrestrial objects with distinctness and brilliancy. But I should deem it inexpedient to fit up any instrument of this description with specula of a shorter focal distance than 20 or 24 inches. The longer the focal distance the more distinctness may be expected, although the aperture of the speculum should be comparatively small.
The following are some of the properties and advantages peculiar to this construction of the reflecting telescope.
1. It is extremely simple, and may be fitted up at a comparatively small expense. Instead of large and expensive brass tubes, such as are used in the Gregorian and Newtonian construction, little more is required than a short mahogany tube, two or three inches long, to serve as a socket for the speculum, with an arm connected with it about the focal length of the speculum. The expense of small specula, either plain or concave, is saved, together with the numerous screws, springs, &c., for centering the two specula, and placing the small mirror parallel to the large one. The only adjustment requisite in this construction, is that of the eye-tube to the speculum; and, by means of the simple apparatus above described, it can be effected in the course of a few minutes. Almost the whole expense of the instrument consists in the price of the speculum and the eye-pieces. The expense of fitting up the four feet speculum, alluded to above—exclusive of speculum and eye-piece—but including mahogany tube and arm, brass sockets, screws, eye-tube, brass joint, and a cast-iron stand painted and varnished, did not amount to £1 : 8s. A Gregorian of the same size would have required a brass tube at least 4½ feet in length, which would cost 5 or 6 guineas, besides the apparatus connected with the small speculum, and the additional expense connected with the fitting up of the joint and stand requisite for supporting and steadying so unwieldy an instrument. While the one instrument would require two persons to carry it from one room to another, and would occupy a considerable space in an ordinary apartment, the other can be moved, with the utmost ease, with one hand, to any moderate distance, and the space it occupies is extremely small.
2. It is more convenient for viewing celestial objects at a high altitude, than other telescopes. When we look through a Gregorian reflector or an achromatic telescope of 4 or 5 feet in length, to an object elevated 50 or 60 degrees above the horizon, the body requires to be placed in an uneasy and distorted position, and the eye is somewhat strained, while the observation is continued. But when viewing similar objects by the Aerial Reflector, we can either stand perfectly erect, or sit on a chair, with the same ease as we sit at a desk when reading a book or writing a letter. In this way, the surface of the moon or any of the planets, may be contemplated for an hour or two, without the least weariness or fatigue. A delineation of the lunar surface may be taken with this instrument with more ease and accuracy than with any other instrument, as the observer can sketch the outline of the object by one eye on a tablet placed a little below the eye-piece, while the other eye is looking at the object. For the purpose of accommodating the instrument to a sitting or standing posture a small table was constructed, capable of being elevated or depressed at pleasure, on which the stand of the telescope is placed. When the telescope is 4 or 5 feet long, and the object at a very high elevation, the instrument may be placed on the floor of the apartment, and the observer will stand in an erect position.
3. This instrument is considerably shorter than a Gregorian telescope whose mirror is of the same focal length. When an astronomical eye-piece is used, the whole length of the instrument is nothing more than the focal length of the speculum. But a Gregorian whose large speculum is 4 feet focus, will be nearly 5 feet in length, including the eye-piece.
4. The Aerial Reflector far excels the Gregorian in brightness. The deficiency of light in the Gregorians is owing to the second reflection from the small mirror; for it has been proved by experiment that nearly the one half of the rays of light which fall upon a reflecting surface is lost by a second reflection. The image of the object may also be presumed to be more correct, as it is not liable to any distortion by being reflected from another speculum.
5. There is less tremor in these telescopes than in Gregorian Reflectors. One cause, among others, of the tremors complained of in Gregorians is, I presume, the formation of a second image at a great distance from the first, besides that which arises from the elastic tremor of the small speculum, when carried by an arm supported only at one end. But as the image formed by the speculum in the aerial telescope is viewed directly, without being exposed to any subsequent reflection, it is not so liable to the tremors which are so frequently experienced in other reflectors. Notwithstanding the length of the arm of the 4 feet telescope above mentioned, a celestial object appears remarkably steady, when passing across the field of view, especially when it is at a moderate degree of altitude; and it is easily kept in the field by a gentle motion applied to the arm of the instrument.
In prosecuting my experiments in relation to these instruments, I wished to ascertain what effect might be produced by using a part of a speculum instead of the whole. For this purpose, I cut a speculum, three feet in focal length, through the centre, so as to divide it into two equal parts, and fitted up each part as a distinct telescope; so that I obtained two telescopes from one speculum. In this case I found that each half of the speculum performed nearly as well as the whole speculum had done before, at least there appeared to be no very sensible diminution in the brightness of the object, when viewed with a moderate power, and the image was equally accurate and distinct; so that if economy were a particular object aimed at in the construction of these instruments, two good telescopes might be obtained from one speculum; or if a speculum happened to be broken accidentally into large fragments, one or more of the fragments might be fitted up on this principle to serve as a tolerably good telescope.
From the experiments I have made in reference to these instruments, it is demonstrable, that a tube is not necessary in the construction of a reflecting telescope—at least on the principle now stated—whether it be used by day or by night for terrestrial or celestial objects; for I have frequently used these telescopes in the open air in the day time, without any inconvenience from extraneous light. Therefore, were a reflecting telescope of 50 or 60 feet in length to be constructed, it might be fitted up at a comparatively small expence, after the expense of the metallic substances, and of casting, grinding, and polishing the speculum is defrayed. The largest instrument of this description which has hitherto been constructed is the 40 feet reflector of Sir W. Herschel. This complicated and most unwieldy instrument had a tube of rolled or sheet iron 39 feet 4 inches in length, about 15 feet in circumference, and weighed about 8000 pounds. Now, I conceive that such enormous tubes, in instruments of such dimensions, are altogether unnecessary. Nothing more is requisite than a short tube for holding the speculum. Connected with one side of this tube (or with both sides were it found necessary), two strong bars of wood, projecting a few feet beyond the speculum end, and extending in front as far as the focal length of the mirror, and connected by cross bars of wood, iron or brass—would be quite sufficient for a support to the eye-piece, and for directing the motion of the instrument. A telescope of 40 or 50 feet in length, constructed on this plan, would not require one fifth of the expense, nor one fourth of the apparatus and mechanical power for moving it to any required position, which were found necessary in the construction of Sir W. Herschel’s large reflecting telescope. The idea here suggested will perhaps be more readily appreciated by an inspection of fig. 71, where A is the short tube, BC and DE the two large bars or arms, connected with cross bars, for the purpose of securing strength and steadiness. At I and K, behind the speculum, weights might be applied, if necessary, for counterbalancing the lever power of the long arm. F represents the position of the eye-piece, and GH the joint and part of the pedestal on which the instrument is placed. With regard to telescopes of smaller dimensions, as from 5 to 15 feet in focal length—with the exception of the expense of the specula and eye-pieces—they might be fitted up for a sum not greater than from 3 to 10 or 15 guineas.
figure 71.
Were any person to attempt the construction of those telescopes, it is possible he might not succeed in his first attempts without more minute directions than I have yet given. The following directions may perhaps tend to guide the experimenter in adjusting the eye-tube to the speculum, which is a point that requires to be particularly attended to, and on which depends the accurate performance of the instrument. After having fixed the eye-piece nearly in the position it should occupy, and directed the instrument to a particular object, look along the arm of the telescope, from K (fig. 69.) to the extremity of the eye-piece at H, and observe, whether it nearly coincides with the object. If the object appear lower than this line of vision, the eye-piece must be lowered, and if higher, it must be raised, by means of the nuts and screws at gd and fe, till the object and the line of vision now stated nearly coincide. The eye-piece should be directed as nearly perpendicular to the front of the speculum as possible, but so that the reflected image of one’s head from the mirror shall not interfere to obstruct the rays from the object. An object may be seen with an approximate degree of distinctness, but not accurately, unless this adjustment be pretty accurately made. The astronomical eye-pieces used for these telescopes are fitted with a brass cap which slides on the end next the eye, and is capable of being brought nearer to or farther from the first eye-glass. In the centre of this cap, next the eye, is a small hole, about the 1/40th or 1/50th of an inch diameter, or about as wide as to admit the point of a pin or a moderate-sized needle. The distance of this hole from the lens next the eye must be adjusted by trial, till the whole field of view appear distinct. A common astronomical eye-piece, without this addition, does not answer well. I find by experience, that terrestrial eye-pieces, such as those used in good achromatic telescopes, are, on the whole, best adapted to this construction of a reflecting telescope.
I have sometimes used these instruments for the purpose of viewing perspective prints, which they exhibit in a beautiful and interesting manner. If a coloured perspective be placed at one end of a large room or gallery, and strongly illuminated either by the sun or by two candles, and one of the reflectors furnished with a small magnifying power, placed at the opposite end of the room—the representation of a street or a landscape will be seen in its true perspective, and will appear even more pleasant and interesting than when viewed through the common optical diagonal machine. If an inverting eye-piece be used—which is most eligible in this experiment—the print, of course, must be placed in an inverted position.
That reflecting telescopes of the descriptions now stated are original in their construction, appears from the uniform language of optical writers, some of whom have pronounced such attempts to be altogether impracticable. Sir David Brewster, one of the latest and most respectable writers on this subject, in the ‘Edinburgh Encyclopedia’ art optics, and in the last edition of his appendix to ‘Ferguson’s Lectures,’ has the following remarks:—‘If we could dispense with the use of the small specula in telescopes of moderate length, by inclining the great speculum, and using an oblique, and consequently a distorted reflection, as proposed first by La Maire, we should consider the Newtonian telescope as perfect; and on a large scale, or when the instrument exceeds 20 feet, it has undoubtedly this character, as nothing can be more simple than to magnify, by a single eye-glass, the image formed by a single speculum. As the front view is quite impracticable, and indeed has never been attempted in instruments of a small size, it becomes of great practicable consequence to remove as much as possible, the evils which arise from the use of a small speculum,’ &c.
The instruments now described have effectuated, in some degree, the desirable object alluded to by this distinguished philosopher, and the mode of construction is neither that of Sir W. Herschel’s front view, nor does it coincide with that proposed by La Maire, which appears to have been a mere hint that was never realized in the construction of reflecting telescopes of a small size. The simplicity of the construction of these instruments, and the excellence of their performance, have been much admired by several scientific gentlemen and others to whom they have been exhibited. Prior to the description of them in the Edin. Philos. Journal, they were exhibited in the Calton Hill Observatory, Edinburgh, in the presence of Professor Wallace, and another gentleman, who compared their performance with that of an excellent Gregorian. As this instrument is distinguished from every other telescope, in being used without a tube, it has been denominated ‘The aerial reflector.’
SECT. 4.—EARL OF ROSSE’S REFLECTING TELESCOPES.
This nobleman, unlike many of his compeers, has, for a considerable number of years past, devoted his attention to the pursuits of science, and particularly to the improvement of reflecting telescopes. He is evidently possessed of high mathematical attainments, combined with an uncommon degree of mechanical ingenuity. About 14 or 15 years ago, he engaged in various experiments with the view of counteracting the effects of the spherical aberration of the specula of reflecting telescopes—which imperfection, if it could be completely remedied, would render the reflecting telescope almost a perfect instrument, as it is not affected by the different refrangibility of the rays of light. His method, we believe, consisted in forming a large speculum of two or three separate pieces of metal, which were afterwards accurately combined into one—a central part which was surrounded by one or two rings ground on the same tool. When the images formed by the separate pieces, were made exactly to coincide, the image of the object towards which the whole speculum was directed, was then found to be as distinct as either image had been when separate. But at the period referred to, a sufficient number of experiments had not been made to determine that his lordship had completely accomplished the object he intended.
Great interest, however, has of late been excited by the improvements which his lordship has made in the formation of specula. Sir W. Herschel never made public the means by which he succeeded in giving such gigantic developement to the reflecting telescope: and therefore the construction of a large reflector has been considered as a perilous adventure. But, according to a report of Dr. Robinson of Armagh, to the Irish academy, the Earl of Rosse has overcome the difficulties which have hitherto been met with, and carried to an extent which even Herschel himself did not venture to contemplate, the illuminating power of this telescope, along with a sharpness of definition little inferior to that of the achromatic; and it is scarcely possible, he observes, to preserve the necessary sobriety of language in speaking of the moon’s appearance with this instrument, which Dr. Robinson believes to be the most powerful ever constructed. The difficulty of constructing large specula, and of imparting to them the requisite degree of polish, has hitherto been considered so great, that from 8 to 12 inches diameter has been in general their utmost size. Indeed, except with the greatest reluctance, London opticians would not accept of orders for specula of more than 9 inches in diameter. It appears, however, that the Earl of Rosse has succeeded, by a peculiar method of moulding, in casting object-mirrors of true speculum metal of three feet in diameter, and of a weight exceeding 17 cwt. He is about to construct a telescope, the speculum of which is six feet in diameter, fifty feet focal distance, and of the weight of four tons; and from what he has already accomplished, it is not doubted that he possesses the power to carry his design into effect. These great masses of metal, which, in the hands of all other makers of specula would have been as untractable as so much unannealed flint-glass, the Earl of Rosse has further succeeded in bringing to the highest degree of polish, and the utmost perfection of curvature by means of machinery. The process is conducted under water, by which means those variations of temperature, so fatal to the finest specula hitherto attempted, are effectually guarded against. To convince Dr. Robinson of the efficacy of this machinery, the earl took the three feet speculum out of its telescope, destroyed its polished surface, and placed it under the mechanical polisher. In six hours it was taken out with a perfect new surface as bright as the original. Under the old system of hand-polishing, it might have required months, and even years, to effect this restoration. Even before achieving these extraordinary triumphs on the solid substance, his lordship had constructed a six feet reflector by covering a curved surface of brass with squares of the true speculum metal, which gave an immense quantity of light, though subject to some irregularities, arising from the number of joinings necessary in such a mosaic work. Of the performance of his lordship’s great telescope, mounted with this reflector, those who have seen it speak in terms of high admiration; but in reference to the smaller and more perfect instrument, furnished with the solid three feet speculum, the language of the Armagh astronomer assumes a tone of enthusiasm and even of sublimity. By means of this exquisite instrument, Dr. Robinson and Sir J. South, in the intervals of a rather unfavourable night, saw several new stars, and corrected numerous errors of other observers. For example, the planet Uranus, supposed to possess a ring similar to that of Saturn, was found not to have any such appendage; and those nebulæ, hitherto regarded, from their apparently circular outline, as ‘coalescing systems,’ appeared, when tested by the three feet speculum, to be very far indeed from presenting a globular appearance; numerous off-shoots and appendages, invisible by other telescopes, appearing in all directions radiating from their edges. Such discoveries, which reflect great honour on the Earl of Rosse, will doubtless have great effect on the interests of astronomical science.[27]
SECT. 5.—REFLECTING TELESCOPES WITH GLASS SPECULA.
After making a variety of experiments with aerial telescopes constructed of metallic specula of different focal lengths, I constructed a telescope on the same plan, with a concave glass mirror. Having obtained a fragment of a very large convex mirror which happened accidentally to have been broken, I caused the convex side to be foliated, or silverised, and found its focal length to be about 27 inches. This mirror, which was about 5 inches diameter, I placed in one of the aerial reflectors, instead of the metallic speculum, and tried its effects with different terrestrial eye-pieces. With a power of about 35 or 40 times, it gave a beautiful and splendid view of distant terrestrial objects—the quantity of light reflected from them, being considerably greater than when a metallic speculum was used, and they appeared on the whole well-defined. The only imperfection—as I had foreseen—consisted in a double image being formed of objects which were remarkably bright and white, such as a light-house whitened on the outside, and strongly illuminated by the sun. One of the images was bright and the other faint. This was obviously owing to the two reflections from the two surfaces of the mirror—one from the convex silverised side, and the other from the concave side next the eye, which produced the faint image—which circumstance has been generally considered as a sufficient reason for rejecting the use of glass specula in telescopes. But although very bright objects exhibited a double image, almost all the other objects in the terrestrial landscape appeared quite distinct and without any secondary image, so that a common observer could scarcely have noticed any imperfection. When the instrument, however, was directed to celestial objects, the secondary image was somewhat vivid, so that every object appeared double. Jupiter appeared with two bodies, at a little distance from each other, and his four satellites appeared increased to eight. The moon likewise appeared as a double orb, but the principal image was distinct and well-defined. Such a telescope, therefore, was not well-adapted for celestial observations, but might answer well enough for viewing terrestrial objects.
Considering that the injurious effects of the secondary image arose from the images reflected from the two surfaces being formed near the same point, and at nearly the same focal distance, I formed a plan for destroying the secondary image, or at least counteracting its effects, by forming the concavity of the mirror next the eye of a portion of a sphere different from that of the convex side which was silverised, and from which the principal image is formed. But, for a long time, I could find no opticians possessed of tools of a sufficient length of radii for accomplishing my design. At length a London working optician undertook to finish a glass speculum, according to my directions, which were, that the convex surface of the mirror should be ground on a tool which would produce a focal distance by reflection of about 4 feet; and that the concave surface should have its focal distance at about 3 feet 3 inches, so that the secondary image might be formed at about 9 inches, within the focal distance of the silverised side, and not interfere to disturb the principal image. But, either from ignorance or inattention, the artist mistook the radius for the half radius of concavity, and the speculum turned out to be only 23 inches focal distance by reflection. This mirror was fitted up as a telescope, on the aerial plan, and I found, as I expected, the secondary image completely destroyed. It produced a very beautiful and brilliant view of land objects, and even the brightest objects exhibited no double image. The mirror was nearly 5 inches in diameter, but the image was most accurately defined when the aperture was contracted to about 3 inches. It was fitted with a terrestrial eye-piece which produced a magnifying power of about 25 times. When directed to the moon, it gave a very distinct and luminous view of that orb, without the least appearance of a secondary image. But as the focal distance of the speculum was scarcely half the length I had prescribed, I did not apply to it any high astronomical powers; as I find, that these can only be applied with effect, in this construction, to a speculum of a considerable focal length. Happening to have at hand a convex lens 10 feet focal length, and 4 inches in diameter—the one side of which had been ground to a certain degree of concavity—I caused the convex side to be foliated, which produced a focus by reflection, at 13½ inches distant. To this mirror I applied terrestrial powers of 15 and 24, with considerable distinctness. The power of 15 produced a very brilliant and distinct view of land objects. Had the mirror been at least 3 times the focal length, it would have formed an excellent telescope, with the same aperture.
SECT. 6.—A REFLECTING TELESCOPE, WITH A SINGLE MIRROR AND NO EYE-PIECE.
figure 72.
On the same principle as that by which a refracting telescope may be constructed by means of a single lens—as represented fig. 51, (page 234) we may form a telescope by reflection with a single mirror, and without an eye-piece. Let AB, fig. 72, represent a large concave speculum, and C its focus—if an eye be placed at D, about 8 or 10 inches within the focal point C, all the objects in the direction of C, or behind the spectator, will be seen magnified by reflection on the face of the mirror, and strongly illuminated. The magnifying power, in this case, will be nearly in the proportion of the focal length of the mirror to the focal length of the eye for near objects. If for example, the focal distance of the mirror be 8 feet, and the distance from the eye at which we see near objects most distinctly, be 8 inches—the magnifying power will be in the ratio of 8 to 96, or 12 times. I have a glass mirror of this description, whose focal length is 4 feet 8 inches, and diameter 6 inches, which magnifies distant objects about 7 times, takes in a large field of view, and exhibits objects with great brilliancy. It presents a very distinct picture of the moon, showing the different streaks of light and shade upon her surface; and, in some cases, shows the larger spots which traverse the solar disc. This mode of viewing objects is extremely easy and pleasant, especially when the mirror is of a large diameter; and the observer is at first struck and gratified with the novel aspect in which the objects appear.
Were a concave mirror of this description—whether of glass or of speculum metal—to be formed to a very long focus, the magnifying power would be considerable. One of 50 feet focal length, and of a corresponding diameter, might produce a magnifying power, to certain eyes, of about 75 times; and, from the quantity of light with which the object would be seen, its effect would be much greater than the same power applied to a common telescope. Sir W. Herschel states, that, on one occasion, by looking with his naked eye on the speculum of his 40 feet Reflector, without the interposition of any lens or mirror, he perceived distinctly one of the satellites of Saturn, which requires the application of a considerable power to be seen by an ordinary telescope. Such an instrument is one of the most simple forms of a telescope, and would exhibit a brilliant and interesting view of the moon, or of terrestrial objects.
PRICES OF REFLECTING TELESCOPES.
1. Prices as stated by Messrs. W. and S. Jones, Holborn, London.
| £ | s. | |
|---|---|---|
| A 4 feet, 7 inch aperture Gregorian reflector, with the vertical motions upon a new invented principle, as well as apparatus to render the tube more steady for observation, according to the additional apparatus of small speculums, eye-pieces, micrometers, &c. | from 80l. to 120 | 0 |
| Three feet long, mounted on a plain brass stand | 23 | 2 |
| Ditto with rack-work motions, improved mountings and metals | 39 | 18 |
| Two feet long without rack-work, and with 4 magnifying powers, improved | 15 | 15 |
| Ditto improved, with rack-work motions | 22 | 1 |
| Eighteen inch, on a plain stand | 9 | 9 |
| Twelve inch ditto | 6 | 6 |
2. Prices as stated by Messrs. Tulley, Islington.
| £ | s. | |
|---|---|---|
| 1 foot Gregorian Reflector, on pillar-and-claw stand, metal 2½ inches diameter, packed in a mahogany box | 6 | 6 |
| 1½ foot ditto on pillar and claw stand, metal 3 inches diameter, packed in a mahogany box | 11 | 11 |
| 2 feet ditto, metal 4 inches diameter | 16 | 16 |
| Ditto with rack-work motions | 25 | 4 |
| 3 feet ditto, metal 5 inches diameter, rack-work motions | 42 | 0 |
| 4 feet ditto, metal 7 inches diameter, on a tripod stand with centre of gravity motion | 105 | 0 |
| 6 feet ditto, metal 9 inches diameter | 210 | 0 |
| 7 feet Newtonian, 6 inches aperture | 105 | 0 |
| 12 feet ditto, metal 12 inches diameter | 525 | 0 |
3. Prices stated by Mr. G. Dollond, St. Paul’s Church Yard.
| £ | s. | |
|---|---|---|
| Reflecting telescopes 14 inches long, in a mahogany box | 9 | 9 |
| Ditto, 18 inches | 12 | 12 |
| Ditto 2 feet | 18 | 18 |
| Ditto with 4 different powers, and rack-work stand supporting the telescope in the centre of gravity | 36 | 15 |
| Ditto 3 feet, with ditto | 50 | 0 |
4. Prices of single speculums and reflecting telescopes, as made by Mr. Grub, Charlemont Bridge works, Dublin.
| NEWTONIAN TELESCOPES. | GREGORIAN REFLECTORS. | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Diameter in inches. | Focal length in feet. | Price of Mirrors alone. | Price of telescope complete without stand. | Diameter in inches. | Focal length in feet. | Price of Mirrors alone. | Price of telescope complete without stand. | |||||
| £ | s. | £ | s. | £ | s. | £ | s. | |||||
| 6 | 3 | 17 | 10 | 25 | 0 | |||||||
| 7 | 7 | 17 | 10 | 27 | 10 | 7 | 3 | 25 | 0 | 34 | 0 | |
| 9 | 10 | 25 | 0 | 40 | 0 | 9 | 4½ | 35 | 0 | 50 | 0 | |
| 12 | 12 | 60 | 0 | 90 | 0 | 12 | 7 | 70 | 0 | 100 | 0 | |
| 15 | 15 | 120 | 0 | 170 | 0 | 15 | 9 | 150 | 0 | 200 | 0 | |
| 18 | 18 | 200 | 0 | 260 | 0 | 18 | 12 | 240 | 0 | 300 | 0 | |
ON THE EYE-PIECES OF TELESCOPES.
Although the performance of telescopes chiefly depends on the goodness of the object-glass, or the object-speculum of the instrument, yet it is of considerable importance, in order to distinct vision, and to obtain a large and uniformly distinct field of view, that the eye-piece be properly constructed. The different kinds of eye-pieces may be arranged into two general divisions—Astronomical and terrestrial.
1. Astronomical eye-pieces.—The most simple astronomical eye-piece is that which consists of a single convex lens; and when the focal distance of this lens, and that of the object-glass of the instrument is accurately ascertained, the magnifying power may be nicely determined, by dividing the focal length of the object-lens by that of the eye-glass. But, as the pencil of white light transmitted by the object-glass, will be divided by the eye-glass into its component colours, the object will appear bordered with coloured fringes, and the distinctness of vision consequently injured. Besides, the spherical aberration, when a single lens is used, is much greater than when two or more glasses are employed. Hence astronomical eye-pieces are now formed by a combination of at least two lenses.
figure 73.
The combination of lenses now generally used for astronomical purposes, is that which is usually denominated the Huygenian eye-piece, having been first proposed by the celebrated Huygens, as a great improvement on the single lens eye-piece. The following figure (73) represents a section of this eye-piece. Let AB be a compounded pencil of white light proceeding from the object-glass; BF a plano-convex field-glass, with its plane side next the eye-glass E. The red rays of the pencil AB, after refraction would cross the axis in R, and the violet rays in V, but meeting the eye-glass E, the red rays will be refracted to O, and the violet nearly in the same direction, when they will cross each other about the point O, in the axis, and unite. The distance of the two glasses FE, to produce this correction, when made of crown glass, must be equal to half the sum of their focal distances nearly. For example, suppose the focal distance of the largest, or field lens, to be 3 inches, and the focal distance of the lens next the eye, 1 inch, the two lenses should be placed exactly at the distance of 2 inches; the sum of their focal length being 4, the half of which is 2. In other words, the glass next the eye should be placed as much within the focus of the field-glass as is equal to its own focal distance. The focal length of a single lens, that has the same magnifying power as this compound eye-piece—is equal to twice the product of the focal lengths of the two lenses, divided by the sum of the same numbers. Or, it is equal to half the focal length of the field-glass. Thus, in reference to the preceding example, twice the product of the focal length of the two lenses—is equal to 6, and their sum is 4. The former number divided by the latter, produces a quotient of 1½, which is the focal length of a single lens, which would produce the same magnifying power as the eye-piece; and 1½ is just half the focal length of the field-glass. The proportion of the focal lengths of the two lenses to each other, according to Huygens, should be as 3 to 1; that is, if the field-glass be 4½ inches, the eye-glass should be 1½; and this is the proportion most generally adopted. But some opticians have recommended that the proportions should be as 3 to 2. Boscovich recommended two similar lenses; and in this case the distance between them was equal to half the sum of their focal distances, as in the Huygenian eye-piece.
The image is formed at IM, at the focal distance of the lens next the eye, and at the same distance from the field-glass. When distinct vision is the principal object of an achromatic telescope, the two lenses are usually both plano-convex, and fixed with their curved faces towards the object glass, as in the figure. Sometimes, however, they consist of what is called crossed lenses, that is lenses ground on one side to a short focus, and on the other side to a pretty long focus, the sides with the deepest curves being turned towards the object glass. A diaphragm, or aperture of a proper diameter, is placed at the focus of the eye lens, where the image formed by the object-glass falls, for the purpose of cutting off the extreme rays of the field lens, and rendering every part of the field of view equally distinct. This is likewise the form of the eye-piece generally applied to Gregorian reflectors. In short, when accurately constructed, it is applicable to telescopes of every description. This eye-piece, having the image viewed, by the eye behind the inner lens, is generally called the negative eye-piece, and is that which the optical-instrument makers usually supply, of three or four different sizes, for so many magnifying powers, to be applied to different celestial objects, according to their nature or the state of the atmosphere in which they are used.
Ramsden’s eye-piece.—There is another modification of lenses, known by the name of the Positive, or Ramsden’s eye-piece, which is much used in Transit instruments, and telescopes which are furnished with micrometers, and which affords equally good vision as the other eye-piece. In this construction the lenses are plano-convex, and nearly of the same focus, but are placed at a distance from each other less than the focal distance of the glass next the eye, so that the image of the object viewed is beyond both the lenses, when measuring from the eye. The flat faces of the two lenses are turned into contrary directions in this eye-piece—one facing the object-glass, and the other the eye of the observer; and as the image formed at the focus of the object-glass, lies parallel to the flat face of the contiguous lens, every part of the field of view is distinct at the same adjustment, or, as opticians say, there is a flat field, which, without a diaphragm, prevents distortion of the object. This eye-piece is represented in fig. 74, where AB and CD are two plano-convex lenses, with their convex sides inwards. They have nearly the same focal length, and are placed at a distance from each other, equal to about two thirds of the focal length of either. The focal length of an equivalent single lens is equal to three fourths the focal length of either lens, supposing them to have equal focal distances. This eye-piece is generally applied, when wires of spider’s lines are used in the common focus; as the piece containing the lenses can be taken out without disturbing the lines, and is adjustable for distinct vision; and whatever may be the measure of any object given by the wire micrometer, at the solar focus, it is not altered by a change of the magnifying power, when a second eye-piece of this construction is substituted.
figure 74.
| figure 75. | figure 76. |
 | |
Aberration of lenses.—In connection with the above descriptions, the following statements respecting the spherical aberration of lenses may not be inappropriate. Mr. John Dollond, in a letter to Mr. Short, remarks, that ‘the aberration in a single lens is as the cube of the refracted angle; but if the refraction be caused by two lenses, the sum of the cubes of each half will be ¼ of the refracted angle, twice the cube of 1 being ¼ the cube of 2. So three times the cube of 1 is only one ninth of the cube of 3.’ &c. Hence the indistinctness of the borders of the field of view of a telescope is diminished by increasing the number of lenses in an eye piece. Sir J. Herschel has shown that if two plano-convex lenses are put together as in fig. 75, the aberration will be only 0.2481, or one fourth of that of a single lens in its best form. The focal length of the first of these lenses, must be to that of the second as 1 to 2.3. If their focal lengths are equal, the aberration will be 0.603, or nearly one half. The spherical aberration, however, may be entirely destroyed by combining a meniscus and double convex lens, as shown in fig. 76, the convex sides being turned to the eye when they are used as lenses, and to parallel rays, when they are used as burning glasses. Sir J. Herschel has computed the following curvatures for such lenses.
| Focal length of the convex lens | +10.000 |
| Radius of its first surface | +5.833 |
| Radius of its second surface | -35.000 |
| Focal length of the meniscus | +17.829 |
| Radius of its first surface | +3.688 |
| Radius of its second surface | +6.294 |
| Focal length of the compound lens | +6.407 |
On the general principles above stated, a good astronomical eye-piece may be easily constructed with two proper lenses, either according to the plan of Huygens or that of Ramsden; and, from what has been now stated it is demonstrably certain, that, in all cases where two glasses are properly combined, such an eye-piece is superior to a single lens, both in point of distinctness, and of the enlargement of the field of view. I lately fitted up an eye-piece, on Ramsden’s principle, with two lenses, each about 3 inches focal length, and 1⅜ inch diameter, placed at half an inch distant, with their convex surfaces facing each other as in fig. 74, which forms an excellent eye-piece for an achromatic telescope, 6 feet 8 inches focal distance, and 4 inches aperture, particularly for viewing clusters of stars, the Milky Way, and the large nebulæ. The field of view is large, the magnifying power is only between 50 and 60 times, and the quantity of light being so great, every celestial object appears with great brilliancy, and it is in general much preferable, when applied to the stars than any of the higher powers. When applied to Presepe in Cancer, it exhibits that group at one view, as consisting of nearly a 100 stars which exhibit a beautiful and most striking appearance.
It may appear a curious circumstance that any eye-piece which is good with a short telescope, is also good with a long one, but that the reverse is not true; for it is found to be more difficult to make a good eye-piece for a short than for a long focal distance of the object-glass.
Celestial eye-pieces are sometimes constructed so as to produce variable powers. This is effected by giving a motion to the lens next the eye, so as to remove it nearer to or farther from the field lens; for at every different distance at which it is placed from the other lens, the magnifying power will either be increased or diminished. The greatest power is when the two lenses are nearly in contact, and the power diminishes in proportion to the distance at which the glass next the eye is removed from the other. The scale of distance, however, between the two lenses, cannot be greater than the focal distance of the field, or inner glass; for if it were, the lenses would no longer form an eye-piece, but would be changed into an inverting opera-glass. For effecting the purpose now stated, the eye-glass is fixed in a tube which slides upon an interior tube on which is marked a scale of distances, corresponding to certain magnifying powers; and, in this way an eye-piece may be made to magnify about double the number of times, when the lenses are in one position than when they are in another—as, for example, all the powers from 36 to 72 times may be thus applied, merely by regulating the distance between the two lenses. When the glasses are varied in this manner the eye-piece becomes sometimes a positive eye-piece, like Ramsden’s, and sometimes a negative one like that of Huygens.
Diagonal eye-pieces. The eye-pieces to which we have now adverted, when adapted to refracting telescopes, both reverse and invert the object, and therefore are not calculated for showing terrestrial objects in their natural position. But as the heavenly bodies are of a spherical form, this circumstance detracts nothing from their utility. When the celestial object, however, is at a high altitude, the observer is obliged to place his head in a very inconvenient position, and to direct his eye nearly upwards; in which position he cannot remain long at ease, or observe with a steady eye. To remedy this inconvenience, the diagonal eye-piece has been invented, which admits of the eye being applied at the side—or at the upper part of the eye-piece, instead of the end; and when such an eye-piece is used, it is of no importance in what direction the telescope is elevated, as the observer can then either sit or stand erect, and look down upon the object with the utmost ease. This object is effected by placing a flat piece of polished speculum-metal at an angle of 45 degrees in respect to the two lenses of the eye-piece, which alters the direction of the converging rays, and forms an image which becomes erect with respect to altitude, but is reversed with respect to azimuth;—that is, in other words, when we look down upon the objects in the field of view, they appear erect; but that part of an object which is in reality on our right hand appears on our left; and if it be in motion, its apparent is opposite to its real motion; if it be moving towards the west, it will seem to move towards the east.
There are three situations in which the diagonal reflector in this eye-piece may be placed. It may be placed either 1. before the eye-piece,—or 2. behind it,—or 3. between the two lenses of which the eye-piece consists. The most common position of the reflector is between the lenses; and this may be done both in the negative and the positive eye-pieces; but as the distance between the two lenses is necessarily considerable, to make room for the diagonal position of the reflector, the magnifying power cannot be great; otherwise, a diagonal eye-piece of this construction remains always in adjustment, and is useful in all cases where a high power is not required. The following is a description and representation of a diagonal eye-piece of this kind in my possession.
figure 77.
In fig. 77, AB represents the plano-convex lens next the object, which is about 2 inches in focal length, and ¾ inch in diameter; CD, a plain metallic speculum of an oval form, well polished, and placed at half a right angle to the axis of the tube; and EF another plano-convex lens, about 1½ inch focal distance. The centre of the speculum is about 1¼ inch from the lens AB, and about ½ or 1/3 inch from EF; so that this eye-piece is a positive one, on the principle proposed by Ramsden. The rays proceeding from the lens AB, and falling upon the speculum, are reflected in a perpendicular direction to the lens EF, where they enter the eye at G, which looks down upon the object through the side of the tube. The real size of this eye-piece is much about the same as that represented in the figure. When applied to an achromatic telescope of 44½ inches focal distance it produces a magnifying power of 36 times, and exhibits a very beautiful view of the whole of the full moon. It likewise presents a very pleasing prospect of terrestrial objects, which appear as if situated immediately below us.
figure 78.
Another plan of the diagonal eye-piece is represented in fig. 78, where the speculum is fixed within the sliding tube which receives the eye-piece, or immediately below it. The part of the tube at AB slides into the tube of the telescope, CD is the speculum placed at half a right angle to the axis of the tube, and EF, the tube containing the lenses, which stands at right angles to the position of the telescope, and slides into an exterior tube, and the eye is applied at G. This construction of the diagonal eye-piece may be used with any eye-piece whatever, whether the Huygenian or that of Ramsden. It will admit of any magnifying power, and if several different eye-pieces be fitted to the sliding tube, they may be changed at pleasure. This form of the diagonal eye-piece, I therefore consider as the best and the most convenient construction, although it is not commonly adopted by opticians.
When any of these eye-pieces are applied to a telescope, with the lens E on the upper part of it, we look down upon the object, if it be a terrestrial one, as if it were under our feet. If we turn the eye-piece round in its socket a quarter of a circle towards the left, an object directly before us in the south, will appear as if it were in the west and turned upside down. If, from this position, it is turned round a semicircle towards the right, and the eye applied, the same object will appear as if it were situated in the east, and inverted; and if it be turned round another quadrant, till it be directly opposite to its first position, and the eye applied from below, the object or landscape will appear as if suspended in the atmosphere above us. This eye-piece, therefore, is capable of exhibiting objects in a great variety of aspects, and the use of it is both pleasant and easy for the observer. But there is a considerable loss of light, occasioned by the reflection from the speculum, which is sensibly felt when very high powers are applied; and therefore when very small stars are to be observed, such as some of those connected with double or triple stars, the observer should not study his own ease so much as the quantity of light he can retain with a high power, which object is best attained with an ordinary eye-piece and a telescope of large aperture.
We have said that a diagonal eye-piece may be constructed with a reflector before the eye-piece. In this case, the speculum is sometimes made to slide before the eye at the requisite angle of reclination, in which application each eye-piece must necessarily have a groove to receive it, and the eye must be applied without a hole to direct it, but it may be put on and taken off without disturbing the adjustment for distinct vision, and is very simple in its application. But, on the whole, the form represented in fig. 78, is the most convenient, and should generally be preferred, as any common astronomical eye-piece can be applied to it. I have used a diagonal eye-piece of this kind, with good effect, when a power of 180 has been applied to the sun and other celestial objects.
Instead of a metallic speculum, a rectangular prism of glass is sometimes substituted; for the rays of light are then bent by reflection from the second polished surface, which ought to be dry, and undergo two refractions which achromatise them; and the same effect is thus produced as by polished metal. Ramsden sometimes gave one of the polished faces of a right angled prism a curve, which prism served instead of a lens in an eye-piece, and also performed the office of a reflector. A semi-globe, or what has been called a Bull’s eye, has also been used as a diagonal eye-piece, and when the curve is well-formed, and the glass good, it is achromatic, and is said to perform pretty well, but it is not superior to the forms already described.
SECT. 2.—TERRESTRIAL EYE-PIECES.
When describing the common refracting telescope, (p. 228.) I have noticed that three eye-glasses, placed at double their focal distances from each other, formerly constituted the terrestrial eye-piece, as represented in fig. 47. But this construction, especially for achromatic instruments, has now become obsolete, and is never used, except in small pocket spy-glasses formed with a single object lens. In its place a four glassed eye-piece has been substituted, which is now universally used in all good telescopes, and which, besides improving the vision and producing an erect position of the images of objects, presents a considerably larger field of view. During the progressive stages of improvement made in the construction of erect eye-pieces by Dollond and Ramsden, three, four, and five lenses were successively introduced; and hence, in some of the old telescopes constructed by these artists, we frequently find five lenses of different descriptions composing the eye-piece. But four lenses, arranged in the manner I am now about to describe, have ultimately obtained the preference. In a telescope having a celestial eye-piece of the Huygenian form, the image that is formed in the focus of the object glass, is that which is seen magnified, and in an inverted position; but when a four glassed eye-piece is used, which produces an erect view of the object, the image is repeated, and the second image, which is formed by the inner pair of lenses AB on an enlarged scale, is that which the pair of lenses CD at the eye-end render visible on a scale still more enlarged. The modern terrestrial eye-piece, represented in fig. 79, is, in fact, nothing else than a compound microscope, consisting of an object lens, an amplifying lens, and an eye-piece composed of a pair of lenses on the principle of the Huygenian eye-piece. Its properties will be best understood by considering the first image of an object, which is formed in the focus of the object glass, as a small luminous object to be rendered visible, in a magnified state, by a compound microscope. The object to be magnified may be considered as placed near the point A, and the magnified image at i, which is viewed by the lens D. Hence, if we look through such an eye-piece at a small object placed very near the lens A, we shall find that it acts as a compound microscope of a moderate magnifying power increasing, in some cases, the diameter of the object about 10 times, and 100 times in surface.
figure 79.
In order to distinguish the different lenses in this eye-piece, we may call the lens A, which is next to the first image, the object-lens, the next to it B, the amplifying-lens, the third, or C, the field-lens, and the one next the eye, D, the eye-lens. The first image formed a little before A, may be denominated the radiant, or the object from which the rays proceed. Now, it is well known as a principle in optics, that if the radiant be brought nearer to the lens than its principal focus, the emerging rays will diverge, and, on the contrary, if the radiant be put farther from the lens than its principal focal distance, the emerging rays will converge to a point at a distance beyond the lens, which will depend on the distance of the radiant from the first face of the lens. In this place an image of the radiant will be formed by the concurrence of the converging rays, but in a contrary position; and the length of the image will exceed the length of the radiant in the same proportion, as the distance of the image from the radiant exceeds that of the radiant from the lens. This secondary image of the radiant at i, is not well-defined, when only one lens, as A, is used, owing to the great spherical aberrations, and therefore the amplifying lens is placed at the distance of the shorter conjugate focus, with an intervening diaphragm of a small diameter at the place of the principal focus; the uses of which lens and diaphragm are, first to cut off the coloured rays that are occasioned by the dispersive property of the object lens,—and secondly, to bring the rays to a shorter conjugate focus for the place of the image, than would have taken place with a single lens having only one refraction. As the secondary image is in this way much better defined and free from colouration, the addition of this second lens is a great improvement to vision. For this reason I am clearly of opinion, that the object glass of a compound microscope, instead of consisting of a small single lens, should be formed of two lenses on the principle now stated, which would unquestionably add to the distinctness of vision.
With respect to the proportions of the focal lengths of the lenses in this four glass eye-piece, Mr. Coddington states, that if the focal lengths, reckoning from A to D, fig. 79, be as the numbers 3, 4, 4 and 3, and the distances between them on the same scale, 4, 6, and 5, 2, the radii, reckoning from the outer surface of A, should be thus:—
| A | {First surface | 27 } | nearly plano-convex. |
| {Second surface | 1 } | ||
| B | {First surface | 9 } | a miniscus. |
| {Second surface | 4 } | ||
| C | {First surface | 1 } | nearly plano-convex. |
| {Second surface | 21 } | ||
| D | {First surface | 1 } | double convex. |
| {Second surface | 24 } |
Sir D. Brewster states, that a good achromatic eye-piece may be made of 4 lenses, if their focal lengths, reckoning from that next the object, be as the numbers 14, 21, 27, 32; their distances 23, 44, 40; their apertures 5.6; 3.4; 13.5; 2.6; and the aperture of the diaphragm placed in the interior focus of the fourth eye-glass, 7. Another proportion may be stated:—Suppose the lens next the object A, to be 1⅞ inch focal length, then B may be 2½ inches, C 2 inches, and D 1½; and their distances AB 2½; BC 3⅝; and CD 2⅜. In one of Ramsden’s small telescopes, whose object glass was 8½ inches in focal length, and its magnifying power 15.4, the focal lengths of the eye glasses were A 0.775 of an inch, B 1.025, C 1.01, D 0.79;—the distances AB 1.18, BC 1.83, and CD 1.105. In the excellent achromatic telescope of Dollond’s construction which belonged to the Duc de Chaulnes, the focal lengths of the eye glasses, beginning with that next the object, were 14¼ lines, 19, 22¾, 14; their distances 22.48 lines, 46.17, 21.45, and their thickness at the centre, 1.23 lines, 1.25, 1.47. The fourth lens was plano-convex, with the plane side to the eye, and the rest were double convex lenses. This telescope was in focal length 3 feet 5½ inches.
The magnifying power of this eye-piece, as usually made, differs only in a small degree from what would be produced by using the first or the fourth glass alone, in which case the magnifying power would be somewhat greater, but the vision less distinct, and were the lens next the eye used alone without the field glass, the field of view would be much contracted. Stops should be placed between the lenses A and B, near to B, and a larger one between C and D, to prevent any false light from passing through the lenses to the eye. The more stops that are introduced into a telescope—which should all be blackened—provided they do not hinder the pencils of light proceeding from the object, the better will the instrument perform.
For the information of amateur constructors of telescopes, I shall here state the dimensions of two or three four glassed eye-pieces in my possession, which perform with great distinctness, and present a pretty large field of view. In one of these, adapted to a 44½ inch achromatic, the lens A, next the object, is 1⅞ inch, focal length, and about 1 inch diameter, with the plane side, next the object. The focal length of the lens B 21/10 inches, diameter 7/10 inch, with its plane side next A; distance of these lenses from each other 24/10 inches. Distance of the field lens C from the lens B 5½ inches. The small hole or diaphragm between A and B is at the focus of A, and is about 1/6 inch diameter, and about 3/8 of an inch from the lens B. The field lens C is 2 inches focal length, and 1¼ inch diameter, with its plane side next the eye. The lens next the eye D is 1 inch focal distance, ½ inch diameter, and is distant from the field glass 1¾ inch, with its plane side next the eye. The magnifying power of this eye-piece is equivalent to that of a single lens whose focal length is half an inch, and with the 44½ inch object glass produces a power of about 90 times. The lens next the eye can be changed for another 1⅜ inch focal length, which produces a power of 65; and the two glasses CD can be changed for another set, of a longer focal distance which produces a power of 45 times. The whole length of this eye-piece is 11½ inches.
In another eye-piece, adapted to a pocket achromatic, whose object glass is 9 inches focal length, the lens A is 1 inch focal length, and ½ inch diameter; the lens B 1¼ inch, and ½ inch diameter, their distance 1½ inch, the lens C 11/10 inch focal length, and 5/8 inch diameter; the eye-lens D 5/8 inch focal length, and 3/8 inch diameter; distance between C and D 1⅛ inch. The distance between B and C 1¾ inch. The whole length of this eye-piece is 4½ inches, and its power is nearly equal to that of a single lens of ½ or 6/10 of an inch focal length, the magnifying power of the telescope being about 16 times. Another eye-piece of much larger dimensions, has the lens A of 2½ inches focal length, and ¾ inch diameter: the lens B 2¾ inches focus and 5/8 inch diameter; and their distance 2¾ inches; the lens C 2⅝ inches focus and 1⅛ inch diameter; the lens D 1¾ inch focus and ¾ inch diameter; distance from each other 2¾ inches. The distance between the lenses B and C is 4 inches. The magnifying power is equal to that of a single lens 1⅛ inch focal distance. When applied to an achromatic object glass 6 feet 7 inches focal length, it produces a power of about 70 times. This eye-piece has a moveable tube 9 inches in length in which the two lenses next the eye are contained, by pulling out which, and consequently increasing the distance between the lenses B and C, the magnifying power may be increased to 100, 120 or 140, according to the distance to which this moveable tube is drawn out. It has also a second and third set of lenses, corresponding to C and D of a shorter focal distance, which produce higher magnifying powers on a principle to be afterwards explained.
Description of an eye-piece, &c. of an old Dutch Achromatic Telescope.
About twenty or thirty years ago, I purchased, in an optician’s shop in Edinburgh, a small achromatic telescope, made in Amsterdam, which was supposed, by the optician, to have been constructed prior to the invention of achromatic telescopes by Mr. Dollond. It is mounted wholly of brass, and in all its parts is a piece of beautiful and exquisite workmanship, and the utmost care seems to have been taken to have all the glasses and diaphragms accurately adjusted. The object glass is a double achromatic, 6½ inches focal distance and 1 inch diameter, but the clear aperture is only 7/8 inch diameter. It is perfectly achromatic, and would bear a power of 50 times, if it had a sufficient quantity of light. The following inscription is engraved on the tube adjacent to the object glass:—“Jan van Deyl en Zoon Invenit et Fecit, Amsterdam, Ao. 1769.” Although Dollond exhibited the principle of an achromatic telescope, eight or ten years before the date here specified, yet it is not improbable that the artist whose name is here stated, may not have heard of Dollond’s invention; and that he was really, as he assumes, one of the inventors of the achromatic telescope. For, the invention of this telescope by Dollond was not very generally known, except among philosophers and the London opticians, till a number of years after the date above stated. Euler, in his “Letters to a German Princess”—in which telescopes are particularly described, makes no mention of, nor the least allusion to the invention of Dollond, though this was a subject which particularly engaged his attention. Now, these letters were written in 1762, but were not published till 1770. When alluding to the defects in telescopes arising from the different refrangibility of the rays of light, in Letter 43, and that they might possibly be rectified by means of different transparent substances, he says, ‘But neither theory nor practice have hitherto been carried to the degree of perfection necessary to the execution of a structure which should remedy these defects.’ Mr. B. Martin, in his ‘Gentleman and Lady’s Philosophy,’ published in 1781, alludes to the achromatic telescope, but speaks of it as it were but very little, if at all superior to the common refracting telescope. And therefore, I think it highly probable that Jan van Deyl, was really an inventor of an achromatic telescope, before he had any notice of what Dollond and others had done in this way some short time before.
But my principal object in adverting to this telescope, is to describe the structure of the eye-piece, which is a very fine one, and which is somewhat different from the achromatic eye-piece above described. It consists of four glasses, two combined next the eye, and two next the object. Each of these combinations forms an astronomical eye-piece nearly similar to the Huygenian. The lens A, next the object, fig. 80, is 5/8 inch focal distance, and 4/10 inch diameter; the lens B 3/8 inch focus, and 1/5 inch diameter, and the distance between them somewhat less than 5/8 inch; the diameter of the aperture e about 1/15 of an inch. This combination forms an excellent astronomical eye-piece, with a large flat field, and its magnifying power is equivalent to that of a single lens 5/8 or 6/8 focal length. The lens C is ½ inch focal length, and 4/10 inch diameter; the lens D ¼ inch focus, and about 1/5 inch diameter; their distance about ½ inch, or a small fraction more. The hole at d is about 1/20 or 1/25 of an inch diameter, and the distance between the lenses B and C about 1½ inch. The whole length of the eye-piece is 3¼ inches—exactly the same size as represented in the engraving. Its magnifying power is equal to that of a single lens ¼ inch focal length; and consequently the telescope, though only 9½ inches long, magnifies 26 times, with great distinctness, though there is a little deficiency of light when viewing land objects, which are not well illuminated.
figure 80.
The glasses of this telescope are all plano-convex, with their convex-sides towards the object—except the lens D, which is double convex, but flattest on the side next the eye, and they are all very accurately finished. The two lenses C and D form an astronomical eye-piece nearly similar to that formed by the lenses A and B. The focus of the telescope is adjusted by a screw, the threads of which are formed upon the outside of a tube into which the eye-piece slides. The eye-piece and apparatus connected with it, is screwed into the inside of the main tube, when not in use, when the instrument forms a compact brass cylinder 6 inches long, which is enclosed in a fish-skin case, lined with silk velvet, which opens with hinges.
The lenses in the eye-pieces formerly described, though stated to be plano-convexes, are for the most part crossed glasses, that is ground on tools of a long focus on the one side, and to a short focus on the other. The construction of the eye-piece of the Dutch telescope above described, is one which might be adopted with a good effect in most of our achromatic telescopes; and I am persuaded, from the application I have made of it to various telescopes, that it is even superior, in distinctness and accuracy, and in the flatness of field which it produces to the eye-piece in common use. The two astronomical eye-pieces of which it consists, when applied to large achromatic telescopes, perform with great accuracy, and are excellently adapted for celestial observations.
SECT. 3.—DESCRIPTION OF THE PANCRATIC EYE-TUBE.
From what we have stated, when describing the common terrestrial eye-piece now applied to achromatic instruments, (p. 349, fig. 79.), it appears obvious, that any variety of magnifying powers, within certain limits, may be obtained by removing the set of lenses CD, fig. 79, nearer to or farther from the tube which contains the lenses A and B, on the same principle as the magnifying power of a compound microscope is increased by removing the eye-glasses to a greater distance from the object-lens. If then, the pair of eye-lenses CD be attached to an inner tube that will draw out and increase their distance from the inner pair of lenses, as the tube a b c d, the magnifying power may be indefinitely increased or diminished, by pushing in or drawing out the sliding tube, and a scale might be placed on this tube, which, if divided into equal intervals, will be a scale of magnifying powers, by which the power of the telescope will be seen at every division, when the lowest power is once determined.
Sir David Brewster, in his ‘Treatise on New Philosophical instruments,’ Book i. chap. vii. page 59, published in 1813, has adverted to this circumstance, in his description of an ‘Eye-piece wire micrometer,’ and complains of Mr. Ezekiel Walker, having in the ‘Philosophical Magazine’ for August, 1811, described such an instrument as an invention of his own. Dr. Kitchener some years afterwards, described what he called a Pancratic or omnipotent eye-piece, and got one made by Dollond, with a few modifications different from that suggested by Brewster and Walker, which were little else than cutting the single tube into several parts, and giving it the appearance of a new invention. In fact, none of these gentlemen had a right to claim it as his peculiar invention, as the principle was known and recognised long before. I had increased the magnifying powers of telescopes, on the same principle, several years before any of these gentlemen communicated their views on the subject, although I never formally constructed a scale of powers. Mr. B. Martin, who died in 1782, proposed many years before, such a moveable interior tube as that alluded to, for varying the magnifying power.
In order to give the reader a more specific idea of this contrivance, I shall present him with a figure and description of one of Dr. Kitchener’s Pancratic eye-pieces, copied from one lately in my possession. The following are the exact dimensions of this instrument, with the focal distances, &c. of the glasses, &c. of which it is composed.
| In. | Tenths. | fig. 81. | |
| Length of the whole eye-piece, consisting of four tubes, when fully drawn out, or the distance from A to B. fig. 81. | 14 | 4 |  |
| Length of the three tubes on which the scale is engraved, from the commencement of the divisions at B to their termination at C. | 9 | 15 | |
| Each division into tens is equal to 3-10ths of an inch. | |||
| When the three inner tubes are shut up to C, the length of the eye-piece is exactly | 5 | 5 | |
| When these tubes are thus shut up, the magnifying power for a 3½ feet achromatic is 100 times, which is the smallest power. When the inner tube is drawn out ⅓ of an inch, or to the first division, the power is 110, &c. | |||
| Focal distance of the lens next the object | 1 | 0 | |
| Breadth of Ditto. | 0 | 65 | |
| The plane side of this glass is next the object. | |||
| Focal distance of the second glass from the object | 1 | 5 | |
| This glass is double and equally convex, Breadth | 0 | 5 | |
| Distance between these two glasses | 1 | 7 | |
| Focal distance of the third or field lens, which is plane on the side next the eye | 1 | 1 | |
| Breadth of Ditto. | 0 | 55 | |
| Focal distance of the lens next the eye | 0 | 6 | |
| Breadth | 0 | 43 | |
| This glass is plane on the side next the eye. | |||
| Distance between the third and fourth glasses. | 1 | 1 | |
From the figure and description, the reader will be at no loss to perceive how the magnifying power is ascertained by this eye-piece. If the lowest power for a 44 inch telescope be found to be 100, when the three sliding tubes are shut into the larger one, then by drawing out the tube next the eye 4 divisions, a power of 140 is produced; by drawing out the tube next the eye its whole length, and the second tube to the division marked 220, a power of 220 times is produced, and drawing out all the tubes to their utmost extent, as represented in the figure, a power of 400 is obtained. These powers are by far too high for such a telescope, as the powers between 300 and 400 can seldom or never be used. Were the scale to begin at 50, and terminate at 200, it would be much better adapted to a 3½ feet telescope. Each alteration of the magnifying power requires a new adjustment of the eye-piece for distinct vision. As the magnifying power is increased, the distance between the eye-glass and the object-glass must be diminished. Dr. Kitchener says, that ‘the pancratic eye tube gives a better defined image of a fixed star, and shows double stars decidedly more distinct and perfectly separated than any other eye tube, and that such tubes will probably enable us to determine the distances of these objects from each other, in a more perfect manner than has been possible heretofore.’ These tubes are made by Dollond, London, and are sold for two guineas each. But I do not think they excel, in distinctness, those which are occasionally made by Mr. Tulley and other opticians.