A Brew­ster-type stere­o­scope is a refract­ing hand-held stere­o­scope intro­duced by David Brew­ster in 1849. The device was lat­er devel­oped into a view­er with an enclosed hous­ing. This mod­i­fi­ca­tion occurred large­ly in response to the use of daguerreo­type stere­oviews, which exhib­it­ed high sur­face reflec­tiv­i­ty and there­fore required shield­ing from ambi­ent light. A hinged lid was added to the top of the view­er in order to direct inci­dent light onto the daguerreo­type and, sub­se­quent­ly, onto paper stere­oviews. Jules Duboscq lat­er incor­po­rat­ed a ground glass ele­ment to enable illu­mi­na­tion of glass stere­oviews by trans­mit­ted light.

(from: Com­pendi­um of Stereoscopes)

The illus­trat­ed view­er also fea­tures an ear­ly lens-tube focus­ing mech­a­nism, patent­ed by Antoine Jules Mar­tin and Alex­is Fay in 1855. To syn­chro­nize the move­ment of both lens tubes, they are con­nect­ed by a met­al piece inside the view­er. This ele­ment was lat­er moved to the exte­ri­or, direct­ly behind the ocu­lar rings, result­ing in the wide­ly adopt­ed design of lat­er box-type viewers.