Active Deposits of Radium, Ihorium, and Actinium. 357 



•so small that it may possibly be due to some extraneous 

 effect due to beating etc. 



From these experiments we may therefore safely conclude 

 that neither tbe black pigment nor the retina as a whole is 

 photo-electric to visual light. It is possible however that, 

 as previously mentioned/ the nul effects obtained with the 

 retina may be due to absorption of the electrons in the 

 •surface. It remains quite conceivable that the rhodopsin 

 in the eye is actually photo-electric, but so immersed in 

 inactive material that the electrons cannot escape. Thus 

 the photo-electric theory of vision, while still presenting 

 many attractions, cannot be said to be in any way confirmed 

 by these experiments, but neither can it be actually refuted 

 by them. To obtain really conclusive evidence it would be 

 ■necessary to isolate sufficient rhodopsin in a pure state to 

 be able to test it directly. 



In conclusion I wish to express my thanks to Professor 

 Pringle and Dr. Fearon for their kind aid in the preparation 

 of the frogs' retinas ; also to Dr. Joly, to whose suggestion 

 and assistance the research is mainly due. 



Iveagh Geological Laboratory, 

 November 1920. 



XXXII. The Distribution of the Active Deposits of Radium, 

 Thorium, and Actinium in Electric Fields. By G. H. 

 Briggs, B.Sc, Lecturer in Physics at the University of 



Sydney *. 



I. Introduction. 



fl^HE experiments to be described in this paper were 

 X begun with the object of deciding whether, as E. M. 

 Wellish | had concluded, there is a definite limiting fraction 

 of the recoil atoms from radium emanation, positivelv 

 charged at the end of the recoil path, or whether, as 

 Gr. H. Henderson concluded, all the recoil atoms from 

 thorium and radium emanations are positively charged at 

 the end of the recoil path. As the work progressed, it 

 was found necessary to extend its scope. 



In his experiments Wellish used a cylindrical vessel 

 with a central electrode. He obtained the following results 

 for the recoil atoms of radium emanation : 8<S'2 per cent. 



* Communicated by the Author. 



t Wellish, Phil. Mag-, xxviii. p. 417 (1914). 



Phil. Mag. S. 6. Vol 41. No. 243. March 1921. 2 B 



