The stere­o­scope is acti­vat­ed by coin inser­tion. The mech­a­nism is locked by default and is unlocked by insert­ing a coin through the top. The slides can then be viewed sequen­tial­ly by rotat­ing the knob on the right side. One slide hold­er includes an exten­sion; after a full rota­tion of all slides, this exten­sion releas­es the coin, which falls to the bot­tom and returns the mech­a­nism to the locked posi­tion. A met­al spring assists in both lock­ing and unlocking.
Slides are insert­ed into the hold­ers by remov­ing the front pan­el of the device. This arrange­ment sim­pli­fies load­ing com­pared with more com­mon designs that require open­ing a lid on the top.

(from: Com­pendi­um of Stereoscopes)

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