This stereographic photo viewer, or stereoscope, is on display at the Stevenson House in Monterey, California. Robert Louis Stevenson lived there in the latter part of 1879, and the house was one of several that my niece and I saw during our tour of the adobe buildings in the city last December.
The Stereograph was an early attempt to create a 3D effect from photographs. Wikipedia provides this helpful description of how they worked:
A typical stereoscope provides each eye with a lens that makes the image seen through it appear larger and more distant and usually also shifts its apparent horizontal position, so that for a person with normal binocular depth perception the edges of the two images seemingly fuse into one “stereo window”. . . . A divider or other view-limiting feature is usually provided to prevent each eye from being distracted by also seeing the image intended for the other eye.
This picture shows the placement of the pictures which are just slightly different from each other,
and this picture shows the visual effect of viewing through the stereoscope.