Many photographic pictures, whether Talbotype or Daguerreotype, are
taken double for the “stereoscope” (fig. 5). This is an instrument
having two lenses (one for each eye), through which the two views
may be seen. The principle is this:--Every object is seen by each
eye in a slightly different view; for instance, if you look at the
edge of a print--placing the printed part towards your right hand,
and bringing the edge in front of the lace between the two eyes--by
closing the right eye, the left will see the back of the print only,
but by closing the left eye the front of the print may be seen, and
the front only. The stereoscope presents both of these views to the
eyes at once, as when we look with both eyes, and the objects have
therefore the appearance of projecting, as do read objects. To take
these views, the slide after one view is taken is moved about four
inches on one side, and then the other is taken; the distance moved
corresponds to the distance between the eyes, and therefore produces
the correct view for each. The box for the stereoscopic slide is
shown at fig. 6, in which A is the first view to be taken, while the
other half of the glass is kept dark, at B, and C is the shutter for
A.