Three photo-electric effects for certain samples of ${\mathrm{Cu}}_2$O and Se as functions of wave-length.—Light from a tungsten lamp or iron arc was dispersed by a quartz spectrometer. Measurements with a thermocouple enabled all results to be reduced to unit light intensity. The observations agree well with previous results obtained with various samples. (1) Decrease of resistance was measured for the range 3900 to 6000 A for ${\mathrm{Cu}}_2$O, maximum at 4600 A, and for 4300 to 9300 A for Se, maximum at 7000 A. (2) Photo-electrolytic potential was found to exist for both back (contact with air) and front (contact with distilled water) illumination, and was measured in the case of front illumination for the range 2700 to 5500 A for ${\mathrm{Cu}}_2$O (maximum at 3500 A, upper limit about 5600 A), and for 2700 to 9000 A for Se (maximum at 5700 A, upper limit about 9500 A). The effect was positive and proportional to the intensity $I$ for ${\mathrm{Cu}}_2$O and to $I^{\frac{2}{3}}$ for Se. (3) Photo-electric wave-length limit was found by the use of absorption screens to be at about 2570 A for ${\mathrm{Cu}}_2$O and at 2260 A for Se. For both ${\mathrm{Cu}}_2$O and Se, all samples showed the same photo-electric limits and those which showed a change of resistance also showed the photo-electrolytic effect, but not vice-versa.

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