An instrument for viewing a stereograph - a pair of pictures, one of which is to be seen only by the left eye and the other only by the right eye. The illusions of 3D solidity and depth perception are provided by the two slightly different views visible to each eye.
One interesting application of stereoscopy (beyond it's entertainment value) is detection of comets. Two pictures of the night sky, pointing at the same angle relative to the Earth's surface, are taken at different times. If the camera is pointed near the ecliptic, the relative motion of all the stars is almost parallel. The two pictures are put under a stereoscope and adjusted so the stars appear to be at infinity. A comet, or planet, or anything that is close enough to the Earth or has enough speed relative to the Earth, will have moved noticably against the backdrop of stars between the two exposures - and will appear in front of (or behind!) the stars. The hard-wiring in the brain for stereoscopic perception is advanced enough that, according to reports, anything that is moving relative to the stars instantly jumps out of the picture in a very distinctive and recognisable way.