William's lantern for color vision testing

Very rare.
In the second half of the 1800′s several spectacular train wrecks led to the discovery that some of the people responsible for these trains were color blind. A lot of interest developed in testing for color vision.
In 1899 Williams of Boston constructed a lantern designed to test the color vision of railway men. In the same year he sent one to Snellen.
The instrument consists of a casing, similar to a railway lantern, containing a candle holder. The candle flame is behind an opening at the front. Also at the front there are a rotating disc with colored glass plates and a second rotating disc with diaphragms of different sizes. By rotating the discs it is possible to turn all possible combinations into position in front of the illuminated opening.