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THE POPULAR SCIENCE MONTHLY. 



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 



Fig. 1. 



Fig. 2. 



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 per- 

 fectly 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 



Fig. 3. 



