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stereoscopic effect if they hold a suitable eyeglass before one eye.
A great hindrance to viewing stereoscopic pictures on paper was the shape of the Brewster stereoscopic box, which is closed all round and only open at the top. This aperture only admits an insufficient amount of light to the picture, which is commonly left in the shade on one side.
This defect was removed in the American stereoscope, which dispenses with any box. Lenses are fitted in a frame g g (fig. 70), which may be held by a handle ; the partition b serves to separate the field of view of the two lenses. The object is placed on the cross-board d d, and this board can be easily slid to and fro, so that the proper position of the
image with reference to the eye may be found.
But the American stereoscope is only suitable for paper pictures. The beautiful transparent stereoscope pictures on glass can only be viewed with stereoscopes of the Brewster type, as they must be seen by transmitted light, and all front light excluded, or the effect will be destroyed.
Double pictures drawn by the hand can also be viewed through the stereoscope. It is evident that the preparation of such pictures only succeeds in very simple subjects. It would be very difficult to represent stereoscopically a complicated object — for example, a man, a landscape, or a machine. Photography, which can produce with the greatest ease pictures of the most complicated objects from any point preferred, rendered this possible. It is only since the invention of photography that the stereoscope, which was formerly a philosophical instrument, has become a favourite instrument with the public. Not
Fig. 70.