The stere­o­scope is rel­a­tive­ly large and fea­tures a focus­ing knob on the under­side. After loos­en­ing the knob, focus is adjust­ed by slid­ing the stere­oview hold­er. The lens pan­el is posi­tioned deep with­in the device, while the wood­en hous­ing reduces inci­dent light. Large side shields fur­ther lim­it stray light and improve view­ing conditions.
This con­fig­u­ra­tion is par­tic­u­lar­ly suit­ed to view­ing autochromes, which are typ­i­cal­ly dark and require strong illu­mi­na­tion. Unwant­ed inci­dent light and reflec­tions can inter­fere with the view­ing expe­ri­ence, and the design address­es these constraints.
(from: Com­pendi­um of Stereoscopes)

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