The dis­tinc­tive fea­ture of this stere­o­scope is that the tray, which holds 25 glass stere­oviews, remains fixed in posi­tion while the images are viewed. A turn­ing knob is locat­ed on the side. By rotat­ing this knob clock­wise or anti­clock­wise, an image is select­ed. In doing so, the entire view­ing assem­bly, includ­ing lens­es and tray, moves. On the top of the view­er, an index with 25 num­bers indi­cates the select­ed image, accom­pa­nied by a push button.
Inside, beneath the tray, is a mech­a­nism with 25 fin­gers. When the but­ton on top is pressed, the fin­ger cor­re­spond­ing to the select­ed image is acti­vat­ed and push­es the slide upwards into view­ing posi­tion before the lens­es. Releas­ing the but­ton returns the image back into the tray.
(from: Com­pendi­um of Stereoscopes)

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