Page:Popular Science Monthly Volume 21.djvu/58

50 efforts of the untrained observer, will generally converge to a single point of fixation. Brewster's mode of preventing this was, like Elliot'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 different pairs of eyes. They served the double purpose of holding the semi-lenses, with edges toward each other, at the most convenient distance



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 between the observer's pupils. On ordinary stereographs, however, three inches is the usual limit. Another office performed by the semi-lenses