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.