Recent Progress in Theoretical Physics. ^ 255 



fundamental equation with which we began this article, and 

 which the experiments of Verdet so remarkably corroborate. 



" The disturbance which constitutes light, whatever its physical 

 nature may be, is of the nature of a vector, perpendicular to the 

 direction of the ray. This is proved from the fact of the interfer- 

 ence of two rays of light, which, under certain conditions, pro- 

 duces darkness, combined with the fact of the non-interference of 

 two rays polarized in planes perpendicular to to each other. For, 

 since the interference depends on the angular position of the planes 

 of polarization, the disturbance must be a directed quantity or 

 vector, and since the interference ceases when the planes of polar- 

 ization are at right angles, the vector representing the disturbance 

 must be perpendicular to the line of intersection of these planes, 

 that is, to the direction of the ray. 



The disturbance, being a vector, can be resolved into compo- 

 nents parallel to x and y, the axis of z being parallel to the direc- 

 tion of the ray. Let ^ and 57 be these components ; then, in the case 

 of a ray of homogeneous circularly-polarized light, 



^ — rcos ^, -rj = r sin d, (1) 



where d ~ nt ~ qz -^ a. (2) 



In these expressions, r denotes the magnitude of the vector, and 

 d the angle which it makes with the direction of the axis of x. 



The periodic time, r, of the disturbance is such that 



n- = 27r. (8) 



The wave-length, X, of the disturbance is such that 



qX = 2;r. (4) 



The velocity of propagation is — 



The phase of the disturbance when t and z are both zero is a. 

 The circularly-polarized light is right-handed or left-handed 

 according as q is negative or positive. 



polarized light transmitted through magnetized glass, parallel to the lines 

 of magnetizing force, with the same quality, right-handed always or left- 

 handed always, is propagated at different rates, according as its course is in 

 the direction or is contrary to the direction in which a north magnetic pole 

 is drawn ; but I believe it can be demonstrated that no other explanation of 

 that fact is pogsible. Hence it appears that Faraday's optical discovery 

 affords a demonstration of the reality of Ampere's explanation of the ulti- 

 mate nature of magnetism." — Sir Wm. Thompson. 



