PROCEEDINGS 



OF THE 



Citmbritrgc pjilasopljtnil jftofrietj. 



January 26, 1891. 

 Prof. G. H. Darwin, President, in the Chair. 



The following Communications were made to the Society : 



(1) On the Electric Discharge through rarefied gases without 

 electrodes. By Prof. J. J. Thomson. 



A vacuum tube was exhibited in which an electric discharge 

 was induced by passing the discharge of Leyden jars through a 

 thread of mercury contained in a glass tube coiled four times 

 along it. The induced discharge was found to be confined to the 

 part of the vacuum tube which was close to the primary discharge, 

 and it did not shew striae. 



It was also demonstrated that an ordinary striated discharge 

 is strikingly impeded by the presence of a strong field of magnetic 

 force. 



(2) The Laws of the Diffraction at Caustic Surfaces. By J. 

 Larmor, M.A., St John's College. 



1. One of the most striking phenomena in connection with 

 the propagation of light or other undulations is the circumstance 

 that under certain conditions, common in optics and easily realis- 

 able in the case of superficial water waves with varying depth of 

 water, there exists a geometrical boundary beyond which the 

 undulations cannot penetrate at all, but in the neighbourhood of 

 which the disturbance is very much intensified. 



In the first approximations of Geometrical Optics, where the 

 undulations are treated as a system of rays, and the energy is 

 considered to be propagated along them, the caustic or envelope 

 of the rays appears as a surface of infinitely great concentration 

 of energy, which is also the boundary of the space into which the 

 energy can penetrate. The conception that must replace this in 



VOL. VII. PT. iv. 12 



