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THE POPULAR SCIENCE MONTHLY. 



efforts of the untrained observer, will generally converge to a single 

 point of fixation. Brewster's mode of preventing this was, like El- 

 liot's, to cause each of the two pictures to be viewed at the bottom of 

 a box, through which light was transmitted. His stereoscope is shown 

 in Fig. 7, which has been taken from an instrument brought to New 

 York in 1850, and much prized by its owner as the first stereoscope 

 ever seen in America. The box is of mahogany, and provided with a 

 lid which can be raised so that an opaque card also may be viewed, if 

 desired, by reflected light admitted from above. The bottom is made 

 of roughened glass so as to diffuse the light that is transmitted, in case 

 a photograph on glass is employed. In either case, the picture can 

 slide easily in and out. To secure the natural convergence of visual 

 lines, a condition which Brewster thought indispensable, a pair of semi- 

 lenses were inclosed in brass tubes at the top of the box. These tubes 

 could be drawn slightly out, like those of an opera-glass, and one was 

 capable of slight lateral motion, being fixed upon a sliding plate of 

 wood as shown in the drawing. They could thus be adapted to differ- 

 ent pairs of eyes. They served the double purpose of holding the 

 semi-lenses, with edges toward each other, at the most convenient dis- 



Fig. 10. The American Grandfather, 1861. 



tauce from the stereograph, and of hiding from each eye the picture 

 intended for the other. Since the rays in transmission are deviated 

 toward the thicker part of the glass, it is possible without discomfort 

 to use pictures on which the stereographic interval exceeds that be- 

 tween the observer's pupils. On ordinary stereographs, however, three 

 inches is the usual limit. Another office performed by the semi-lenses 



