A stereoscope is an optical instrument with two eyepieces used to impart a three-dimensional effect to two photographs of the same scene taken at slightly different angles.

Antique stereograph cards:

same image as above viewed through a Stereograph:  
(squint for better effect)

my drawing of a stereoscope:

Stereoscope is also the title of a drawn film by artist William Kentridge, created by drawing, erasing and redrawing each movement and transition.  You can see the video here.

Stills from William Kentridge's film Stereoscope: