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 1/2 inch focal length, and 4/10 inch diameter; the lens D 1/4 inch focus, and about 1/5 inch diameter; their distance about 1/2 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 1-1/2 inch. The whole length of the eye-piece is 3-1/4 inches--exactly the same size as represented in the engraving. Its magnifying power is equal to that of a single lens 1/4 inch focal length; and consequently the telescope, though only 9-1/2 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.
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.