Joe Lipka , Apr 06, 2001; 10:36 a.m.
I am not familiar with this specific piece of equipment, but have used others similar to it. It acts as an exposure meter for your enlarger. For a specific negative and paper combination, you can calibrate both the highlights and shadows of a test negative on your enlarger easel. Then load any other negative (of the same film type)into your enlarger, and read the highlights and shadows of that negative on the easel. You then adjust the enlarger aperture to make the meter read the same as your test negative.
This system works well if you keep using the same film, paper, enlarging times, developer and developing time. The only variable is the enlarger lens aperture.