Altostereoscope

This unmarked box-slid­ing stere­o­scope is prob­a­bly asso­ci­at­ed with C. A. Stein­heil’s Altostereo sys­tem for 9x12cm glass stere­oviews, although cer­tain con­struc­tion details — notably the full-width hinged load­ing flap — are more typ­i­cal of Erne­mann stere­o­scopes. As no mak­er’s label sur­vives, the exact man­u­fac­tur­er remains uncertain.
The term Altostere­o­scope refers to the unusu­al 9x12cm stereo­scop­ic for­mat, in which each of the two stereo­scop­ic half-images is ver­ti­cal­ly ori­ent­ed rather than hor­i­zon­tal, as is the case in most stereo­scop­ic systems.