Instoscope

the extinction meter image by Mario Groleau (Image rights)

Drem Instoscope image by Anssi Puisto (Image rights)

The Drem Instoscope was an extinction meter in. When viewing thru the ocular one could see a number of letters. The least visible letter was explained on the table which was wrapped around the instrument. The table showed the possible shutter-speed/aperture combinations for the least visible letter.