On Huyghens's Principle applied to Physical Optics. 243 



other arrangements than those which correspond to the maxi- 

 mum of effect, there is no constant relation between different 

 voltaic combinations; we can only compare them, and judge 

 of their relative preferableness by referring them to this maxi- 

 mum of action. We have, by eliminating z, the equations 



C (max) = 



2/\L 2 s/ V L ' 

 whence we deduce, by substituting the numerical values above 



found for A, A', X, V s' = s. 0'06 



and with reference to the number of pairs z' = z . 0*6; 

 that is to say, it requires only a pile of 6 square feet of 

 platina to replace a pile o/'lOO square feet of copper \ or with 

 reference to the number of pairs, 6 pairs of * platina, each of 

 a square foot of surface, xvill produce the same effect as 10 pairs 

 of copper, each of which presents a surf ace of 10 square feet. 

 This eminent superiority of platina, as in Mr. Grove's com- 

 bination, is verified by many experiments on a large scale. 



XXXVI. On the Application of Huyghens's Principle in 

 Physical Optics. By R. Potter, Esq., B.A.* 



TN the present paper I propose to examine some of the con- 

 sequences of the method at present followed in developing 

 the results of the undulatory theory of light, which consists 

 in considering elementary waves, having their origin in some 

 previous positions of the main waves, as the cause of these 

 latter in succeeding positions. In the Memoires de V Acad, 

 for 1821 and 1822, Fresnel announces this method in the 

 following terms : — 



M Application du Principe d'Huygens aux Phenomenes de 

 la Diffraction. 



" Ce principe que me parait une consequence rigoureuse 

 de l'hypothese fondamentale, peut s'enoncer ainsi : Les vibra- 

 tions d'une onde lumineuse dans chacun de ses poiiits peuvent 

 etre regardees comme la somme des mouvemens elementaires qu'y 

 enverraient au meme instant, en agissant isolement, toutes les 

 parties de cette onde consideree dans une quelconque de ses 

 positions anterieures." 



Again, in speaking of the analytical process, he says, 

 " La recherche de la loi suivant laquelle leur intensite varie- 

 rait autour de chaque centre d'ebranlement, presenterait sans 

 doute de grandes difficultes ; mais heureusement nous n'avons 

 pas besoin de la connaitre ; car il est aise de voir que les effets 



* Communicated by the Author, 

 R2 



