392 On the Experimental Relations [1857. 



approach more and more to the truth of nature, and in the 

 fulness of time coincide with it. 



The phenomena of light itself are, however, the best and 

 closest tests at present of the undulatory theory ; and if that 

 theory is hereafter to extend to and include other actions, the 

 most effectual means of enabling it to do so will be to render 

 its application to its own special phenomena clear and sufficient. 

 At present the most instructed persons are, I suppose, very far 

 from perceiving the full and close coincidence between all the 

 facts of light and the physical account of them which the theory 

 supplies. If perfect, the theory would be able to give a reason 

 for every physical affection of light ; whilst it does not do so, 

 the affections are in turn fitted to develope the theory, to extend 

 and enlarge it if true, or if in error to correct it or replace it by 

 a better. Hence my plea for the possible utility of experiments 

 and considerations such as those I am about to advance. 



Light has a relation to the matter which it meets with in its 

 course, and is affected by it, being reflected, deflected, trans- 

 mitted, refracted, absorbed, &c. by particles very minute in 

 their dimensions. The theory supposes the light to consist of 

 undulations, which, though they are in one sense continually 

 progressive, are at the same time, as regards the particles of 

 the ether, to and fro transversely. The number of progressive 

 alternations or waves in an inch is considered as known, being 

 from 37,600 to 59,880, and the number which passes to the eye 

 in a second of time is known also, being from 458 to 727 billions ; 

 but the extent of the lateral excursion of the particles of the 

 ether, either separately or conjointly, is not known, though both 

 it and the velocity are probably very small compared to the 

 extent of the wave and the velocity of its propagation. Colour 

 is identified with the number of waves. Whether reflexion, 

 refraction, &c., have any relation to the extent of the lateral 

 vibration, or whether they are dependent in part upon some 

 physical action of the medium unknown to and unsuspected by 

 us, are points which I understand to be as yet undetermined. 



Conceiving it very possible that some experimental evidence 

 of value might result from the introduction into a ray of separate 

 particles having great power of action on light, the particles 

 being at the same time very small as compared to the wave- 

 lengths, I sought amongst the metals for such. Gold seemed 



