﻿70 Mr. W. B. Croft : Some 



mechanical question in a previous paper *, where it was shown 

 (§§ 10, 11) that, granted the fundamental assumption and an 

 infinite propagation-velocity for gravitational stress, we may 

 construct an atom having a finite number of freedoms. But in 

 whatever way mechanical degradation of energy were elimi- 

 nated, the difficulty of electromagnetic degradation would 

 also have to be met, and without the doctrine laid down in 

 Theorem I. there appeared to me to be no means of escape. 

 Without making any assumption as to the constitution of a 

 molecule or the nature of a collision, we may admit that in 

 any body not absolutely cold there are particles in relative 

 motion, so that two neighbouring particles are sometimes 

 nearer together and sometimes farther apart. To realize the 

 intermittence of contact required by Theorem II., we have 

 only to suppose that when (but not until) the proximity of 

 two particles has reached a certain limit electrification is 

 capable of passing freely from one to the other. 



The question of perfect or imperfect conductivity in the 

 ultimate particles of bodies must be of importance in relation 

 to the constitution of matter and its connexion with the 

 ether ; and whether or not the demonstrations above can be 

 generally accepted as conclusive, the subject is certainly one 

 which will repay further investigation. 



V. Some Observations on Diffraction. 

 By W. B. Ckoft, M.A.\ 

 [Plates I.-IV.] 



IT is proposed to illustrate various forms of this phenomenon 

 by photographs % produced directly from the wave- 

 interference. 



After the inauguration of the idea about 1665 by Grimaldi, 

 Hooke, and Huygens, there was little progress, either in 

 extended observation or in philosophical grasp of the prin- 

 ciples, until the beginning of this century. Since that time 

 the subject has been treated in two ways. 



1st. The Diffraction of Fraunhofer and Schwerd. 



This kind is familiar to many through the observations 

 of Sir John Herschel of Diffraction in a Telescope. It is 

 sometimes described as the Diffraction from Parallel Light. 



* " A Theory concerning the Constitution of Matter," Phil. Mag. 

 February 1892, p. 191. 



t Communicated by the Physical Society : read January 26, 1894. 



X It is not convenient to reproduce all the photographs: the selected 

 figures 2, 3, 4, 10, 12, 13, 71, 72, 75, 83 will be found on Plates I.-IV. 



