Page:Popular Science Monthly Volume 24.djvu/98

88 and free from specks, and, in handling them, touch only their edges. Remember, also, that the double-convex lens must be outside when the telescope is fitted up. Have ready a strip of tissue-paper, just the width of the thickness of the lenses at the edges: gum this on one side, and, holding the two lenses together with the fingers of the left



hand, wind the strip around the edges, so as to fix them together, and thus make a single piece which can be easily handled. When this is dry, take a strip of brown paper one and a quarter inch wide, and with paste form a short tube or cell, C, around the object-glass, using (say) five thicknesses. Fig. 2 shows the object-glass and cell in section.

To form the eye-piece: cut off a portion of the smallest tube—that on which the draw-tube was rolled—one and three-eighths inch in length, and make the ends even and square. Make, now, two disks of blackened cardboard, of the diameters respectively of seven-eighths inch and one inch. Punch or cut out exactly in the center of each disk an aperture one quarter inch in diameter. Gum the edges of the smaller disk, and fit it into the tube, exactly three quarters of an inch from one end, and, of course, five eighths of an inch from the other end. Then take the two-inch plano-convex lens, and, having made it perfectly clean, cement it on to the end of the tube nearest the perforated disk, with the plane surface inward. Use shellac varnish, or gold-size, for cementing the lens on to the edge of the tube. Cement the three-quarters inch plano-convex on to the one-inch perforated disk, centrally over the aperture, and with the plane surface next the card. When the cement on both lenses is dry, which will be in a day or two, fasten this one-inch disk to the open end of the tube, keeping the lens inside. A single layer of tissue-paper, gummed on to the out-side of the tube, and turned down about one sixteenth of an inch all around the edge of the two-inch lens, and around the disk at the other end, will now serve as a sort of fastener to both, and will complete the eye-piece, which is shown in full size in section, Fig. 3. The smaller lens a must be next the eye when the telescope is fitted up; the larger lens b, called the field-glass, will be inside and facing the object-glass.

For fitting together the various parts now completed few directions