Stereoscope ‘à deux foyers’

The Stéréo­scope à deux foy­ers (dual-focus stere­o­scope), in the 6×13 for­mat, was intro­duced by Mat­tey as a nov­el­ty in 1922 to sat­is­fy enthu­si­asts who wished to view both black-and-white stereo­scop­ic images and Autochromes. While black-and-white stere­o­graphs work very well with a short focal length and strong mag­ni­fi­ca­tion, which enhances them per­fect­ly, the Autochrome, because of its trichrome fil­ter, is dis­played much bet­ter with a stere­o­scope hav­ing a long focal length and low mag­ni­fi­ca­tion, which lessens the enlarge­ment of the col­ored starch grains.
Since a medi­um-focus stere­o­scope was not sat­is­fac­to­ry either for black-and-white images or for Autochromes, Mat­tey mar­ket­ed this dual-focus stere­o­scope. Its reversible body can be detached or cou­pled at the front in the same way as a cam­era mag­a­zine, allow­ing the focal length of the two sets of eye­pieces — fit­ted with achro­mat­ic lens­es and eye-cups — to be adjust­ed. The eye-cups them­selves are mount­ed quick­ly by sim­ple fric­tion fitting.
The front part of the stere­o­scope is iden­ti­cal to the stan­dard mod­el, with focus­ing adjust­ment and vari­able eye spac­ing. The cab­i­net work is in old mahogany with oxi­dized met­al fit­tings. The set includes the stere­o­scope and a dou­ble set of eye­piece assemblies.
Note: the eye­piece assem­blies with the ‘large brass fit­tings’ con­tain the­long-focus lens­es; con­verse­ly, the short-focus lens­es are mount­ed in the eye­pieces with the ‘small brass fittings’.
(from: Moulin­ier et al. His­toires de vision­neuses stéréo­scopiques français­es. Limo­ges, 2025, p. 104. Author’s translation.)