402 Prof. Faraday on the Experimental Relations 
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, bemg reflected, deflected, transmitted, 
refracted, absorbed, &c. by particles very minute in their dimen- 
sions. 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, moving 
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 whe- 
ther 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 mtroduction 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 
especially fitted for experiments of this nature, because of its 
comparative opacity amongst bodies, and yet possession of a real 
transparency ; because of its development of colour both in the 
reflected and transmitted ray ; because of the state of tenuity and 
division which it permitted with the preservation of its integrity 
as a metallic body ; because of its supposed simplicity of cha- 
racter ; and because known phznomena appeared to indicate 
that a mere variation in the size of its particles gave rise to a 
variety of resultant colours. Besides, the waves of light are so 
large compared to the dimensions of the particles of gold which 
in various conditions can be subjected to a ray, that it seemed 
probable the particles might come into effective relations to the 
much smaller vibrations of the ether particles ; in which case, if 
reflexion, refraction, absorption, &c. depended upon such rela- 
tions, there was reason to expect that these functions would 
change sensibly by the substitution of different-sized particles of 
