898 



SCIENCE. 



[N. S. Vol. IV. No. 103. 



angle of incidence constant they investigated 

 the change in the actino- electric current as 

 the polarizing nicol was rotated. Attempts 

 were made to use a cell containing a plane- 

 parallel quartz window, but no cement 

 could be found which would hold and yet 

 not alter chemically the sensitive surface. 

 Cells could, however, be constructed which 

 would remain tight for a short time, and 

 served to check the results obtained with 

 spherical glass cells. In using the latter 

 cells precautions were taken to have the 

 rays from the zircon lamp pass normally 

 through the glass. Only the central por- 

 tion of the liquid metal surface was used. 



105. Indicating the angle through which 

 the nicol was turned from the position of 

 maximum action by x, the actino-electric 

 current was found to be given by the equa- 

 tion : 



I = A cos^ X + B sin^ x. 



This formula agrees with the assumption 

 that the actino-electric effect is proportional 

 to the incident light, but that the propor- 

 tionality factor is different for light polar- 

 ized in the plane of incidence and perpen- 

 dicular thereto. 



106. Observations were made at several 

 different angles of incidence (70°, 66°, 40°, 

 23 ° ) . The ratio of A to B was found to de- 

 pend upon the angle of incidence. Both A 

 and B were small for normal incidence. A 

 increases to a maximum at an incidence of 

 about 60°, and then decreases. B appears 

 to decrease steadily to practically zero at 

 grazing incidence. At 60° the ratio of B 

 to A was found to be 50. 



107. Good effects with polarized rays 

 could only be obtained with the smooth, 

 nearly plane surface of liquid K or Na. 

 Solid Na or K always show rather a rough 

 surface and behave in practically the same 

 manner for polarized and unpolarized light. 



108. Experiments were tried with amal- 

 gamated zinc at ordinary pressures, but 



here, too, the behavior with polarized rays 

 was scarcely different from that with un- 

 polarized light. The authors think that 

 this is due to the difficulty of obtaining 

 ultra-violet rays completely polarized. 



EFFECT OF LIGHT UPON POSITIVELY CHARGED 

 BODIES. 



109. When the attempt is made to find a 

 satisfactory explanation of the action of 

 light upon charged bodies, the fact that 

 the action seems to be confined to negative 

 charges is of great significance. Several 

 observers have, indeed, found indications of 

 a discharging action in the case of posi- 

 tively charged bodies. Mention has already 

 been made of such indications in the case 

 of the experiments of Hallwachs, Bighi, and 

 Elster and Geitel. But these observers 

 were able to show that the apparent dissi- 

 pation of a positive charge under the influ- 

 ence of ultra-violet rays was in reality a 

 secondary phenomenon, due to the convec- 

 tive discharge from negative bodies in the 

 neighborhood. They were of the opinion 

 that the action of light upon a positive 

 charge, if such action exists at all, is too 

 small to be measured. 



110. In contradiction to these conclu- 

 sions stand the results of numerous experi- 

 ments by Branly.* Under circumstances 

 where previous observers had found no 

 trace of any action, Branly detected a very 

 rapid dissipation of positive electricity. 

 The lack of a detailed description of the 

 experiments leading to these results makes 

 it impossible to draw any conclusion in re- 

 gard to their reliability. But the question 

 is of such importance that Elster and Geitelf 

 have recently undertaken a series of experi- 

 ments intended either to confirm or dis- 



*C. E. 110, p. 751, 1890; 110, p. 898; 114, p. 68, 

 1892; 116, p. 741, 1893; Lumiere elect. 41, p. 143, 

 1891; Jour, de Phys. 2, p. 300, 1893; Abstracts in the 

 Beiblatter, 



t Wied. Ann. 57, p. 24, 1896. 



