Among the physical methods which have been suggested for the elucidation of chemical problems, the measurement of light-absorption is becoming increasingly important. Opinion is, however, still very divided concerning the utility of the different methods and the measure of trust which can be reposed in the results obtained. The element of uncertainty is so great that the correctness of conclusions based on the measurements is not infrequently questioned, and occasions have arisen in which the investigation of a problem by different methods has led to intrinsically different results. A specially clear case is found in the divergence between the photographic and photoelectric methods for the measurement of light absorption in the ultra-violet. For this reason we have thought it desirable to supplement in one or two directions the investigations of Baly, Morton and Riding on this subject. It must be stated at the outset that in judging the value of the methods under consideration the aim of the proposed investigation must first be ascertained.