Object-glass should be half an in. in diameter, focus 24 in., or as long as convenient. Eye piece may be a single lens of low power with cross spider lines fixed in its focus. The target will then appear inverted. The lenses are inclosed in a brass tube with a hinge Or ball joint at the breech or eye piece end, and slides at the muzzle, to depress the object glass for increased elevation. The two points of attachment to the barrel are the same as for ordinary fore-and-leaf sights,

Telescope, To make a cheap, - A. correspondent says: "1 selected a meniscus 1 in. in diameter and of 48-in. focus. This was for my object-glass. I had already in my possession a two-lensed double-convex jeweler's eye-glass; one of these lenses was used for the eye-piece, its focal length being a trifle over 1 in. The tube was made of pine-wood. A piece of straight, evenly-grained one-in. pine board, 2 in. wide and 8 feet long, was cut in the middle, and the two pieces, after making a tapering semicircular groove in each, well glued together. This done, the next thing was to give it a round, tapering form, 2 in. in diameter at one end, and a trifle over an inch at the other. This was done with a common carpenter's plane. I now had a tube 4 feet long, with a tapering hole through its length, and 1-1/4 in. in diameter at its largest end. Two wooden cells for the lenses were then turned in a lathe, and were made to go on to the tube, as does the cover of a wooden pill-box. A round hole, the size of the lens, was made in each, the meniscus being contracted to 3/4 in., and the eye-glass to 1/4 in. diameter. The piece carrying the eye-glass was made so as to slide some distance on the tube, for adjustment to distinct vision. The tube was painted and varnished, and mounted equatorially; and it proved to be a good instrument, showing Jupiter's moons, their movements and eclipses, handsomely, the ring of Saturn, the horned appearance of Venus, the mountains and craters on the moon, the spots on the sun, etc. Several of the nebulae were also visible, especially those in Andromeda, Orion, Her-eulex, and Sagittarius. The whole need not cost over two dollars, beside the time in making, provided one is a mechanic.

"The meniscus (concave on one side and convex on the other) is the proper form for a single-lens object-glass, and a plano-convex lens makes the best form for the eye-piece. Care must be taken to so set the lenses in their cells that their foci will meet centrally. When this is the case, the lenses are said to be well centred, and in that way we get rid of most of the prismatic color. Another point that wants attention is the mounting. Absolute steadiness is required for close observation. I used to put mine upon a post set firmly in the ground. The equatorial arrangement for mounting is described in nearly every work on telescopes."