470 The Effect of Gravitation on LigJit. 



warming them up, the reddish colour gradually disap- 

 peared with a simultaneous increase in conductivity. On 

 allowing them to cool, they very slowly began to present 

 the former tint, conductivity, too, decrensing at the same 

 time. The second variety of crystals were usually found to 

 be deposited in the hotter part of the tube, and had slight 

 conductivity which did not show any appreciable change on 

 slightly heating. The third variety was primarily non- 

 conducting. The peculiar heat sensitivity of the first 

 variety is therefore quite possibly responsible for the 

 enormous temperature variation of resistance of selenium ; 

 and that something akin to transformation is actually taking 

 place, is suggested from the change of colour on warming. 

 It is hoped that, tine being available, researches will be 

 pursued in this direction. 



6. Summary. 



1. The variation of resistance of selenium with tempera- 

 ture has been carefully determined, and a formula has been 

 proposed to express the said variation. 



2. It has been shown experimentally that not more than 

 1/25 of the "light effect" is attributable to heat produced 

 by light. 



3. A change of colour taking place at the same time as 

 the change in resistance has been observed by warming 

 crystals of selenium produced by the sublimation method. 



4. A modified theory of transformation is suggested to 

 account for the various observations in connexion with the 

 heat-effect. 



In conclusion, I beg to thank Prof. A. 0. Rankine for the 

 kind interest he took in this work and for the facilities given 

 to me in conducting the experiment. 



LIV. The Effect of Gravitation on Light. 

 By Harold Jeffreys, M.A., D.Sc* 



THE query in Sir Oliver Lodge's note (Phil. Mag., June 

 1921, pp. 944-5) concerning the legitimacy of deriving 

 the equations 



dt 7 , 9 d6 , 



7 1 -=k and V s -r- —h 



as as 



for the motion of a light wave is relevant to my paper 

 " On the Crucial Tests of Einstein's Theory of Gravitation " 

 (Monthly Notices of R. A. S., Dec. 1919, pp. 138-154). Of 



* Communicated by the Author. 



