applying to Light under the action of Magnetism, tfl 



union after emergence will have its plane of polarization 

 turned from the original plane of polarization, in one experi- 

 ment in the same direction as the hands of a watch, and in the 

 other experiment in the opposite direction, as referred to the 

 eye of a person looking in the direction of the path of the 

 light. 



This result agrees with the phaenomena of boracic glass, 

 &c. under the action of magnetic forces. 



Instead of making the second term on the right-hand side 



7 rj 



of the equation depend on -j— , we might with equal success 



have adopted -r^gj -7—3 — Tp or an y other differential coeffi- 



cient of an odd order in which the number of differentiations 

 with respect to t is odd. Different powers of t and v will be 

 introduced by different selections. In order to determine 

 which of these selections is best adapted to represent the phae- 

 nomena, it will be necessary to determine the deviation of the 

 plane of polarization for light of different colours. 



If-y— be adopted, the equations suggested by me will 



amount to this : — " The force upon any particle in the direc- 

 tion of one ordinate depends in part upon its velocity in the 

 direction of the other ordinate." There is no insurmountable 

 difficulty in conceiving that this may be true, although we 

 have at present no mechanical reason a priori for believing 

 that it is true. 



To remove the possibility of misunderstanding, I will re- 

 peat that I offer these equations with the same intention with 

 which Prof. MacCullagh's equations were offered , not as giving 

 a mechanical explanation of the phaenomena, but as showing 

 that the phaenomena may be explained by equations, which 

 equations appear to be such as might possibly be deduced 

 from some plausible mechanical assumption, although no such 

 assumption has yet been made. 



I am, Gentlemen, 



„ .a. r, . , Your obedient Servant, 



Royal Observatory, Greenwich, 7 



May 7, 1846. G. B. AlRY. 



