An opera glass is simply a pair of these little telescopes, joined together so that they can be focused at the same time by means of an adjusting screw, as shown in [Fig. 152].
Another simple telescope is formed of two double convex lenses. As in the telescope just described the larger lens, or object glass, is placed in the end of the tube nearest the object to be viewed and the smaller lens, or eyepiece, is placed in the tube nearest the eye.
In this telescope, though, the eyepiece is a double convex lens and while a larger image is formed in the eye it is inverted, or upside down, so that this kind of a telescope is of no use as a glass to spy things with on the surface of the Earth.
How to Make a Cheap Telescope.—Lenses are absurdly cheap. If you live in a large city you will find lens grinders who will sell you the kind of lenses you need for either kind of telescope for a dollar or so.
Telescope No. 1.—This telescope is fashioned after an opera glass, that is, it has a concave lens for an eyepiece.
Fig. 153.—Pasteboard Mounting of Lens.
Fig. 154.—Pasteboard
Lens Mounting.
Get two pasteboard tubes of the kind described for the pinhole telescope; the bore, or hole, of the first tube should be 1¾ inches in diameter and it should be 2 inches long. Have the second tube a little smaller than 1¾ inches in diameter on the outside so that it will slide easily into the larger tube and yet not leak light, and have this tube 1½ inches long. Paint the inside of both tubes with black paint to keep the walls of the tube from reflecting any stray rays of light which may strike them. The outside of both tubes can be covered with bookbinders’ cloth to give them a neat appearance.