Scattering of Positive Electricity by Light. 443 



the surface of the box. If this is covered with bright metal — as 

 tin-foil — the transference of electricity largely increases, and 

 becomes strikingly great if a piece of amalgamated zinc is 

 placed upon it. 



Since Herr Branly gives no information as to the distance 

 of the illuminated plate from the quartz window of the metal 

 box, we may consider it not improbable that this was chosen 

 too small, and that the dispersion of positive electricity 

 observed by him in ultra-violet light was caused by the 

 deceptive action of the light reflected from the surface of the 

 electrified disk. 



We believe that we are justified, by the results of the 

 experiments described, in asserting that an increase in the 

 dispersion of positive electricity by illumination of the elec- 

 trified surface by ultra-violet light has not been proved. 



The striking inability of surfaces of alkali-metal to retain a 

 charge of negative electricity in ordinary light might suggest 

 that a possible action of light with a positive charge might be 

 expected to take place most readily with such surfaces. As 

 we have mentioned in a previous paper *, exhausted glass 

 globes of which the one electrode is formed by an alkali- 

 metal, the other by platinum, also allow a photoelectric 

 current to be observed in more or less distinct manner when 

 they are reversed, i. e. when the alkali-metal forms the 

 positive pole. But we had also arrived at the conclusion that 

 in this case the photoelectric action had its seat not at the 

 surface of the alkali-metal, but at the platinum electrode. 

 There is, in fact, formed upon the platinum a superficial layer 

 by condensation of the vapours of the alkaline metal from 

 which in fight negative electricity passes to the anode. By 

 heating the platinum wire with a galvanic current this layer 

 is volatilized, and the photoelectric cell becomes — in its 

 reversed arrangement — for a short time insensitive to light. 



When we recently repeated this experiment with better 

 arrangements and greater care, we found that after the wire 

 had been heated there remained n small amount of sensitive- 

 ness to light, which, perhaps, had its origin in a scattering of 

 positive electricity from the surface of the alkali-metal. It 

 seemed important to determine the seat of this action without 

 doubt, whether anode or kathode. 



We started from the observation that the current as usually 

 produced by illumination of the kathode is dependent upon 

 the position of the plane of polarization of the incident light 

 with respect to the surface of the kathode f. It was to be 



* Elster and Geitel, Wied. Ann. xliii. p. 236 (1891). 

 f ? Cf. Elster and Geitel, Wied. Ann. Hi. p. 440 (1894). 



