The Stéréo-Project was pre­sent­ed to the French Pho­to­graph­ic Soci­ety dur­ing its meet­ing of Novem­ber 6, 1903. Its oper­a­tion was as follows:
In stereo­scop­ic pro­jec­tions, the two images are pro­ject­ed side by side onto a screen. To per­ceive the stereo­scop­ic effect, each per­son must use a spe­cial view­ing device that allows the two images to be super­im­posed, just as in an ordi­nary stereoscope.
The Demaria Broth­ers Stéréo-Project, in its exter­nal appear­ance, resem­bles a con­ven­tion­al stere­o­scope. How­ev­er, it con­tains two sets of mir­rors. Two fixed mir­rors inclined at 45° and two mov­able mir­rors rotat­ing around two axes, oper­at­ed simul­ta­ne­ous­ly by a met­al knob. At the front are two mov­able diaphragm screens, adjust­ed by a lever, which can be moved clos­er togeth­er or far­ther apart.
Two eye­pieces with­out lens­es serve to guide the observer’s vision. Depend­ing on the dis­tance and posi­tion of the observ­er, and the size of the pro­ject­ed images on the screen, the angle of the mir­rors must be adjust­ed so that the two images over­lap in the viewer’s per­cep­tion, cre­at­ing the stereo­scop­ic effect.
(from: Moulin­ier et al. His­toires de vision­neuses stéréo­scopiques français­es. Limo­ges, 2025, p. 57. Author’s translation.)