A compensating circuit comprising two blocking‐layer photoelectric cells in a parallel connection with a galvanometer and potentiometer‐rheostat is described. Analysis of the circuit shows that a uniform scale attached to the potentiometer‐rheostat may be used to indicate transmission (or reflectance) of light by a sample if the resistance of the potentiometer and the series internal resistances of the photoelectric cells are small in comparison with the parallel internal resistances of the photoelectric cells. The performance of the circuit under different conditions was tested by means of wire screens of known transmission. With moderate illumination on the photoelectric cells and using a 50 ohm potentiometer‐rheostat carrying a scale graduated from 0 to 100, errors in indicating transmission are believed to be not greater than ±0.1 on this scale.