|| These two vision testers are in the Jack Judson Collection
at the Magic Lantern Museum in San Antonio, Texas.
In both cases the testee looked through the hole in the left end of the apparatus, and moved the slider back and forth until the dots on the slider surface.
The upper device is marked "count the dots", and there is a series of dots arranged in an arc on the surface.
The lower device is marked "Property of Self-Test Optical Company of Chicago/Copyright STC 1928, pat. pend."