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THE POPULAR SCIENCE MONTHLY. 



(0'), a point half-way between the objects. Each eye being thus de- 

 ceived, the objects seem to be one. 



This may be easily demonstrated by drawing on the plain back 

 of a stereograph two similar circles, olie with an horizontal and one 

 with a vertical diameter, op^^osite the centres of the pictures, and, 

 half-way between them, a heavy vertical line (as in Fig. 2). Then, 

 looking at the card through the instrument, a circle will appear with 

 crossed diameters, and with a vertical line on each side (as in Fig. 3). 



::----o' 



By closing and opening the eyes alternately, the diameters and 

 vertical lines will appear and disappear, but the circle will remain 

 constant. The right eye is thus deceived into seeing the circle in the 

 middle, and the line away to the left ; and the left is deluded on the 

 other side. The same eflfect may be quickly obtained by sticking a 

 common pin upright on the middle of the rack, with a carpet-tack on 

 each side, when a tack will be visible standing between two pins. 



In selecting a stereoscope, first look that the glasses be large and 

 heavy, and of perfectly clear glass, and see that they are wide enough 



Fig. 3. 



for the eyes to come opposite the middle of them, producing no feel- 

 ing of being shut in by dark objects at the side. Many instrument- 

 makers seem unaware that heads differ in width. 



Then, placing a picture in the rack, see that the diaphragm hides 

 no part of it when the rack is drawn up to a short focus. Near-sighted 

 people have rights as well as others. Should the pictures not appear 

 as one, remove the card and stick up a pin on each side of the rack 

 equidistant from the middle, moving them in or out until they appear 

 as one. If the pins have to be within two or two and a half inches 



