Foe PPL Dot Vibration Test S25, Each

Details

The pair of dots, which appear to merge into a single dot, determines the amount of vibration of the slide in the appropriate axis. 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 0.02 inch, pairs being squispaced 0.25 inch.