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.