Royal Society. 295 



by this simple means that a half vortex-ring moving near 

 another in a parallel path with a less velocity tends to follow 

 in the path of the first ; and that two equal half vortex-rings 

 moving in oj^posite directions along the same path separate 

 into two vortices which move at right angles to the path of 

 the original vortices. 



Whenever a stratum of dense vapour is formed in the higher 

 regions of the atmosphere, it is evident from the preceding 

 discussion that it will tend to descend in the form of a vortex- 

 ring if disturbing forces are not present. The results of this 

 paper are as follows : — 



1. An analogy between the strain -potential and the velocity- 

 potential is indicated. 



2. It is shown that the formation of liquid rings is a neces- 

 sary consequence of the fundamental equations of strain and 

 of hydrodynamics, and that a difference of density is not ne- 

 cessary to produce them. 



3. Vortices can and do arise in certain processes of diffusion. 



4. Simple methods of studying vortex motions in liquids 

 are given. 



Physical Laboratory, Harvard College, 

 Cambridge, U. S. A. 



XLI. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 235.] 

 June 15, 1876. — Dr. J. Dalton Hooker, C.B., President, in the Chair. 



THE following papers were read : — 

 " The Action of Light on Selenium." By Prof. W. Gr. Adams, 

 E.B.S., and Mr. B. E. Day. 



The paper contains an account of a series of experiments which 

 have been carried on during the past year, and which have had for 

 their object the investigation of the electrical behaviour of selenium, 

 especially as regards its sensitiveness to light. The first part con- 

 tains a short summary of the results obtained by Professor Adams, 

 which have been communicated to the Society*. 



It has been already shown that the action is due principally, if 

 not entirely, to those rays of the spectrum which are luminous, 

 and that the ultra-red or the ultra-violet rays have little or no 

 effect ; also that the intensity of the action depends on the illu- 

 minating power of the light, being directly as the square root of 

 that illuminating power. 



It was also observed that with the same piece of selenium at the 

 same temperature, the resistance diminished as the battery-power 

 * See Phil. Mag. 1876, vol. i. pp. 155, 322. 



