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 fhat, 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 ivhat 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 in ces- 



