Ernemann manufactured different stereoscopes to support it's stereo camera portfolio: hand-held stereoscopes for different glass formats, a chain-type revolving viewer and a slide-tray cabinet viewer. The Ernemann Universal Stereoscope is a wooden foldable viewer that supports the formats 45x107mm up to 8,5x17cm. It must have been a useful device for retailers and photographers who had to deal with both paper and glass images and different formats.Unfolded, it consists of two parts that are mounted on a base plate. The front panel contains the lenses, which are optically very good. While focusing, the whole front panel moves forward or backward. The distance between the lenses is adjustable and the set distance is visible in millimetres. The rear part consists of a frosted glass where the stereoviews are placed. Behind the glass, a folding white flap provides a neutral background while viewing the stereoviews. I like the oblique viewing angle of the device as it makes it easier to position your eyes in front of the lenses.
This compact and versatile stereo viewer has also its disadvantages. It's an open system so it's prone to reflections and it lacks the intimate viewing experience of a closed hand-held stereoscope. Another drawback is the small focus knob, which is partly built into the base plate. It contributes to the compact size of the viewer, but it makes focusing more difficult. Finally, the stereoviews should be placed behind a metal clip. Placing thicker glass stereoviews is somewhat cumbersome and you have to be careful not to scratch the glass slides.
It's more useful for paper stereocards. I started to appreciate this viewer when I got some stereocards of Jean Agélou with an unusual size of 9 x 15cm. This viewer is very suitable for these "exotic" formats.