Early 20th century ophthalmoscopy tool with its carrying case. Ophthalmoscopic tools are used to examine the interior of the eye and measure its optical properties. This is a Theilung double prism dating from the 1920s. Double prisms can be used to diagnose certain optical conditions such as strabismus. This particular device was invented by the Swiss ophthalmologist Edmund Landolt (1846-1926). The manufacturer is the F. Fritsch company of Vienna, Austria.