240 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



the incident light, but their velocity is independent of it, as well as 

 of the wave length for any given metal. This odd result does not at 

 all agree with the quanta hypothesis which, according to Einstein, 

 leads to a linear variation of the initial energy mt»^/2 with the fre- 

 quency. We may further in our measures replace the initial velocity 

 by the maximum positive potential V which the metal can take under 

 the influence of the rays (that is, the potential of the stoppage of the 

 electrons). The first measures made upon this matter by Laden- 

 burg^ showed an increase of the initial velocity with the exciting 

 frequency. Taken up by Ladenburg and Marlav,^ Hull,^ Hughes,* 

 Richardson,^ and others, the experiments have confirmed, although 

 not without dispute and difficulty, the qualitative result of Ladenburg 

 and apparently the theoretical law of variation due to Einstein. Cer- 

 tain writers contest this last deduction and claim a parabolic in place 

 of a linear law of variation.^ Our own unpublished experiments com- 

 pleted upon this question lead us to reserve our decision, because of 

 the smallness of the ranges of wave lengths studied by all these 

 experimenters. It will be necessary to take up with quartz appa- 

 ratus this question, working with the alkaline metals from the visible 

 spectrum way up to the extreme ultra-violet. This is the only pro- 

 cedure which will allow a real experimental test of the theory of 

 quanta. We will close with the results obtained by Millikan^ and 

 his pupils, who have found in certain cases abnormally high initial 

 velocities. It looks as if there might be some experimental error due 

 to the mode of production of the discharge by the ultra-violet light 

 and the influence of the electric waves from the source upon the 

 measuring apparatus. 



(2) The discovery of the ionization of gases by ultra-violet light 

 was made by Lenard in 1900. As the effect was produced across 

 several centimeters of air and made very great positive and small 

 negative ions, it was natural to interpret the phenomenon, as did 

 J. J. Thomson, as an Hertz effect upon the solid or liquid particles 

 present in the gas. The researches of Langevin and those of Eugene 

 Bloch ^ have shown, indeed, that the greater part of the Lenard effect 

 is certainly due to this cause. 



The Lenard effect upon the gas itself nevertheless does exist. Re- 

 found by J. J. Thomson^ and then more decisively by Palmer,^° it 



1 Ladenburg, Phys. Zeitschr., vol. 8, p. 590, 1907. 



2 Ladenburg and Markav, Phys. Zeitschr., vol. 9, p. 821, 1908. 



3 Hull, Phys. Zeitschr., vol. 10, p. 537. 



* Hughes, Phil. Mag., vol. 21, p. 393, 1911 : Proc. Cambr., vol. 16, p. 167, 1911. 



^ Richardson, Phil. Mag., vol. 24, pp. 570, 575, 1912. 



6 Kuntz, Cornelius, Phys. Review, 1910 and 1913. 



''Millikan and Wright, Phys. Review, January and February, 1911. 



8 Bloch, Radium, p. 240, 1908. 



8 Thomson, Proc. Cambr., vol. 14, p. 417, 1907. 



10 Palmer, Nature, vol. 77, p. 582, 1908; Phys. Rev., p. 1, 1911. 



