) = 
THEORIES OF LIGHT. 231 
could not dare to appeal to it, so complicated, so minute, 
and so directly designed for the purpose proposed were 
the experimental means employed by Fresnel in the study 
of circular polarization. Perhaps it may be proper to 
observe that the greater part of them were suggested by 
theoretical ideas ; for without that, most of the experi- 
ments of our colleague offer combinations, of which, so to 
speak, it would seem impossible that any one would have 
thought. If, in writing the history of the sciences, it is 
just to put in their full light the discoveries of those who 
have cultivated them with distinction, it is important also, 
—it seems to me right,—though freely stating the truth, 
yet not to put it in such a light as might render it a source 
of discouragement to any who might be engaged in the 
same pursuits. 
PRINCIPAL CHARACTERISTICS OF THE SYSTEM OF EMIS- 
SION AND OF THAT OF WAVES.—GROUNDS ON WHICH 
FRESNEL WAS LED TO REJECT UNRESERVEDLY THE 
SYSTEM OF EMISSION. 
After having studied with so much care the properties 
of luminous rays, it was natural to inquire of what light 
consists? This scientific question, one of the grandest, 
_without contradiction, on which men have ever occupied 
themselves, has given occasion for the most animated dis- 
cussion. Fresnel took an active part in it. I will therefore 
endeavour to point out precisely the nature of the ques- 
tion, and give a concise analysis of the experiments to 
which it has given rise. 
The senses of hearing and smell enable us to discover 
the existence of bodies at a distance by totally different 
means. Every odorous substance undergoes a species of 
evaporation: minute particles are sent off from it inces- 
