Stereoscope ‘Patent Achromatic Mirror Stereoscope’

The Achro­mat­ic Mir­ror Stere­o­scope was spe­cial­ly designed for view­ing paper stere­oviews that were mount­ed in a book. It is an open view­er with a mir­ror on the base of the device, to which it owes its name. Two achro­mat­ic lens­es are placed in adjustable brass tubes which bring the stere­oview in focus.
(from: Com­pendi­um of Stereoscopes)