Prism Tester
Mfr: unknown, probably USA
Date: possibly 1940's
This piece of equipment was purchased from a surplus outfit in California. There are no markings on it but it appears to be a prism tester for 6x30 binoculars, probably from World War II. Mounted on one end are the optics of one half of of a 6x30 binocular with a blank slot for one of the prisms. The prism slot measures 21mm x 43mm. The eyepiece is on a micrometer controlled sliding mount for fine focus and has a simple crosshair reticle. On the other end is a tube enclosing a target pattern with a light bulb for illumination. There are also one or more lenses in the tube. The target pattern consists of a crosshair with nine bull's eyes, one in the center and two in each quadrant. The picture on the right shows the eyepiece crosshair superimposed on the target reticle with a prism from a 6x30 in the unit (sorry for the slightly out of focus image).
I believe that once this unit was properly collimated with a "golden" reference prism, prisms under test were inserted in the slot and checked for deviation of the light path. Note that the slot appears to have a tight fit for American 6x30 binocular prisms from WWII such as for the MK33. I assume that prisms were sorted for the direction and amount of deviation so that they could be matched in a binocular half to cancel the deviation, or rejected if there was too much deviation.