The glass slides are mount­ed in a met­al har­mon­i­ca belt that can hold up to twelve slides. Instruc­tions on how to posi­tion the belt in the device are print­ed on the top. The slides are firm­ly clamped in frames, sug­gest­ing that they were intend­ed to remain per­ma­nent­ly in the belt. Fre­quent replace­ment could eas­i­ly dam­age the glass slides, and addi­tion­al belts were there­fore sold sep­a­rate­ly. This con­struc­tion pro­vides good pro­tec­tion for the frag­ile glass slides.
By depress­ing a small lever on the right side, the hold­er with a slide is released and falls into posi­tion in front of the lenses.
(from: Com­pendi­um of Stereoscopes)