Prism Stereoscope ‘Stéréoscope redresseur, modèle extra-riche’, Luxe

The Stéréo­scope redresseur makes it pos­si­ble to cor­rect­ly view stere­oviews that have not been trans­posed. The view­er achieves this by means of two types of prism lens­es. This com­bi­na­tion pro­duces a lat­er­al cor­rec­tion (left–right rever­sal) of the images. It per­forms the same func­tion as swap­ping the left and right images, but opti­cal­ly rather than physically.

(from: Com­pendi­um of Stereoscopes)