The Focatron.

The Focatron Model C, a simple and novel photoelectric device, measures image sharpness in the focal plane of any copy camera. With a halftone image projected onto the copy and the Focatron probe mounted on the ground glass, the Focatron meter reads maximum when the camera is brought to critical focus - maximum image sharpness - not only more convenient, but far more precise than focusing by eye or with tapes.