ETHER. 765 



are the measures, not of substances, but always of processes taking place in a. 

 substance. We therefore conclude that light is not a substance but a process 

 going on in a substance, the process going on in the first portion of light 

 being always the exact opposite of the. process going on in the other at the 

 same instant, so that when the two portions are combined no process goes 

 on at all. To determine the nature of the process in which the radiation of 

 light consists, we alter the length of the path of one or both of the two 

 portions of the beam, and we find that the light is extinguished when the 

 difference of the length of the paths is an odd multiple of a certain small 

 distance called a half wave-length. In all other cases there is more or less 

 light ; and when the paths are equal, or when their difference is a multiple 

 of a whole wave-length, the screen appears four tunes as bright as when one 

 portion of the beam falls on it. In the ordinary form of the experiment these 

 different cases are exhibited simultaneously at different points of the screen, 

 so that we see on the screen a set of fringes consisting of dark lines at 

 equal intervals, with bright bands of graduated intensity between them. 



If we consider what is going on at different points in the axis of a beam 

 of light at the same instant, we shall find that if the distance between the 

 points is a multiple of a wave-length the same process is going on at the 

 two points at the same instant, but if the distance is an odd multiple of 

 half a wave-length the process going on at one point is the exact opposite 

 of the process going on at the other. - 



Now, light is known to be propagated with a certain velocity (3 < 004xl0 10 

 centimetres per second in vacuum, according to Cornu). If, therefore, we sup- 

 pose a movable point to travel along the ray with this velocity, we shall 

 find the same process going on at every point of the ray as the moving point 

 reaches it. If, lastly, we consider a fixed point in the axis of the beam, we 

 shall observe a rapid alternation of these opposite processes, the interval of time 

 between similar processes being the time light takes to travel a wave-length. 



These phenomena may be summed up in the mathematical expression 



u = A coa(nt-px + a) 



which gives u, the phase of the process, at a point whose distance measured 

 from a fixed point in the beam is x, and at a time t. 



We have determined nothing as to the nature of the process. It may 

 be a displacement, or a rotation, or an electrical disturbance, or indeed any 



