The stere­o­scope and its oper­at­ing prin­ci­ple are unique. The stere­oviews are print­ed on a sin­gle glass plate mea­sur­ing 13 × 18 cm. Each plate con­tains twen­ty-four images arranged as twelve stereo pairs in six hor­i­zon­tal rows. A slid­ing lid at the top of the view­er pro­vides access for load­ing the plate. By oper­at­ing a lever on the right-hand side, a wood­en hold­er is raised, allow­ing the glass plate to be insert­ed from above. Once posi­tioned, the low­est row of stere­oviews becomes vis­i­ble. The lens pan­el can be moved lat­er­al­ly to the left and right to align with the two images of a stereo pair. By low­er­ing the lever, the glass plate descends step­wise, bring­ing the next row into the view­ing position.
(from: Com­pendi­um of Stereoscopes)

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