228 MM. Elster and Geitel on the Electro-optical 



(for example, the difference in pressure) play a part in the 

 phenomenon. 



Further, induced thereto by the attempts of Herr Wanka* 

 to recognize in the well-known Hertzian experiment of the 

 production of an electric spark by the light of another, the 

 influence of the direction of the vibrations of light to the 

 illuminated kathode, we have looked for a dependence of the 

 same phenomenon on the angle of incidence. The experi- 

 mental arrangement was essentially the same as that already 

 described, only that the action of the light was observed not 

 by the fall of potential of an illuminated surface, but by the 

 discharge of the synchronous, so-called passive spark of a 

 second inductorium in similar phase with the first. The con- 

 denser used with the first is not now required, but it is 

 advisable to connect the second with a Leyden jar in order to 

 give greater intensity to the passive spark, and thus to facili- 

 tate its observation. We have not found any perceptible dif- 

 ference in the action which was clearly recognizable at different 

 angles of incidence (0° and 50°), either with the zinc plate as 

 kathode or with the mercury surface. 



It is, however, to be remarked that the Hertzian pheno- 

 menon, the resolution of a spark by the ultra-violet illumina- 

 tion of the kathode, is not to be regarded as altogether the 

 same as the scattering of negative electricity in ultra-violet 

 light observed by Herr Hallwachs. This latter is within 

 wide limits proportional to the intensity of the light, and 

 therefore first disappears completely on excluding the light, 

 whilst with the first, for a given distance apart of the elec- 

 trodes, a finite strength of light is necessary and sufficient. 

 On this account the above described experiment would perhaps 

 have given a positive result if the striking distance of the 

 passive spark had each time been adjusted by a micrometer- 

 screw to its maximum value. 



In regard to the nature of the photo-electric process, the 

 results of the experiments with polarized light on the whole 

 indicate that we have here to do with an immediate action 

 which the light-rays exert by exciting electrical vibrations. 

 In this or a similar sense Herrn Wiedemann and Ebert f , we 

 ourselves J, and Herr Jaumann § have already declared. Yet, 



* J. Wanka, Mitth. d. deutsch. math. Ges. in Prag. p. 63 (1892). 



t E. Wiedemann and H. Ebert, Wied. Ann. xxxiii. p. 263, (1888) ; 

 and xxxv. p. 259 (1888). 



% J. Elster and H. Geitel, W 7 ied. Ann. xli. p. 175 (1890), and xliv. 

 p. 736 (1891). 



§ In the above-cited paper of Wanka, p. 58 (1892) ; also G. Jaumann, 

 Wien, Ber. cxiv., ii a. p. 9 (Jan, 1895). 



