The Auto­mat­ic-inverseur is a device used for the auto­mat­ic trans­po­si­tion of stereo neg­a­tives and the pro­duc­tion of devel­oped pos­i­tive stere­oviews on glass.
The device con­sists of a frame fit­ted with a cov­er enclos­ing the neg­a­tive and pos­i­tive plates. The frame includes an aper­ture through which light rays from an enclosed source are direct­ed onto select­ed areas of the plates. In the stan­dard con­fig­u­ra­tion, the light source is an elec­tric lamp, although a gas burn­er or oil lamp may also be used. With elec­tric illu­mi­na­tion, the light inten­si­ty can be adjust­ed readily.
Spe­cial slid­ing mech­a­nisms, oper­at­ed by knobs posi­tioned on the sides of the frame, move the plates in sequence so that the right-hand por­tion of the pos­i­tive aligns with the left-hand por­tion of the neg­a­tive, and conversely.
A large-scale timer begins auto­mat­i­cal­ly at zero when expo­sure starts and returns to zero after print­ing. This pro­vides a con­sis­tent means of ensur­ing iden­ti­cal expo­sure times for both sec­tions of the plate. A yel­low glass cov­er over the dial allows the dia­pos­i­tives to be exam­ined dur­ing development.
(from: Com­pendi­um of Stereoscopes)