I have found a bunch of academic papers and studies discussing the use of the Glenn Colorule, but can't find even a picture of another out there anywhere. There are records of in a few academic libraries, so I know it was produced by s explained on the box text, it was made to be used to test the color vision of observers, to test the quality of light sources, and/or to demonstrate disagreement caused by "pseudo," or false, color matches. There's a good bit of of writing out there about its use for detecting color blindness, and a number of studies recording differences in perception results among a group of users. It's also just a really cool object, with hand-dyed fabric swatches representing progressions from blue and violet to ochre/yellow-green on scales that slide to allow for a user to make their closest match, and with a piece of board with square cutout to isolate that match. As noted on the box, different dyes were used to create the samples on the two different scales/slides, "and thus tests of various observers under various light sources will result in radical differences of opinion." Quite an interesting piece of optical history will all sorts of applications now, including as a conversation piece and engaging group activity!
14 13/16" x 2 13/16", very good condition both inside and out, sliding mechanism works just as it should.