S25 Foepple Dot Vibration Test Pattern

The amount of vibration of the slide in the appropriate axis is determined by the pair of dots which appear to merge into a single dot.
The pattern on the S25 is an array of 20 pairs of dots converging on a single dot. The distance between each dot pair increases by 0.001 inch to a maximum of 00.2 inches, pairs being equispaced 0.25 inch. Supplied on 76 x 26mm glass slide.