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.