The top of the Stéréothèque can be opened, after which a slide tray is placed on a sled inside the view­er. On the right side there is a crank. When it is turned 180 degrees clock­wise, a slide is lift­ed from the tray by two met­al fin­gers that rise from the bot­tom of the view­er and are attached to a ver­ti­cal­ly mount­ed cir­cu­lar disc. The stere­oview is posi­tioned in front of the lens­es and can be viewed.
When the crank is turned anoth­er 180 degrees, the slide is returned to the tray and the tray advances slight­ly so that the next image is select­ed. At the front there is a small round view­ing win­dow through which the num­ber of the select­ed image can be read. The crank han­dle can be turned in both direc­tions, allow­ing bidi­rec­tion­al navigation.
(from: Com­pendi­um of Stereoscopes)

Fur­ther read­ing: Stere­oscopy His­to­ry Series — Mat­tey (A. Ruiter)