The Collaborative International Dictionary of English v.0.48:

Stereoscope \Ste"re*o*scope\, n. [Stereo- + -scope.]
   An optical instrument for giving to pictures the appearance
   of solid forms, as seen in nature. It combines in one,
   through a bending of the rays of light, two pictures, taken
   for the purpose from points of view a little way apart. It is
   furnished with two eyeglasses, and by refraction or
   reflection the pictures are superimposed, so as to appear as
   one to the observer.
   [1913 Webster]

   Note: In the reflecting stereoscope, the rays from the two
         pictures are turned into the proper direction for
         stereoscopic vision by two plane mirrors set at an
         angle with each other, and between the pictures. In the
         lenticular stereoscope, the form in general use, the
         eyeglasses are semilenses, or marginal portions of the
         same convex lenses, set with their edges toward each
         other, so that they deflect the rays coming from the
         picture so as to strike the eyes as if coming direct
         from an intermediate point, where the two pictures are
         seen apparently as one.
         [1913 Webster] Stereoscopic

WordNet (r) 3.0 (2006):

stereoscope
    n 1: an optical device for viewing stereoscopic photographs