102 THE WAVE THEORY OF LIGHT. 



phenomena presented as objections to the uncliilatoi\y theory are 

 exjilained with marvelous facility, even down to the smallest details. 



I would fain have traced by what an admirable suite of experiment 

 and reasoning- Fresnel arrived at this discovery, one of the most 

 important of modern science; but time presses. 



It has sufficed me to explain how very great the difficulties were 

 which he had to overcome in order to establish it. 



I hasten to point out its consequences. 



You saw, at starting, the purely physiological reasons which make 

 the study of light the necessary center of information gathered by 

 human intelligence. You judge now, b}" the march of this long devel- 

 opment of optical theories, what preoccupations it has always caused 

 to powerful minds interested in natural forces. Indeed, all the phe- 

 nomena which pass before our eyes involve a transmission to a distance 

 of force or movement; let the distance be infinitely great, as in celes- 

 tial space, or infinitely small, as in molecuhir intervals, the mystery is 

 the same. But light is the agent which brings us the movement of 

 luminous bodies. To fathom the mechanism of this transmission is to 

 fathom that of all others, and Descartes had the admirable intuition of 

 this when he comprehended all these problems in a single mechanical 

 conception. Here is the secret bond which has alwaj's attracted the 

 phj'sicists and geometers toward the stud}' of light. Looked at from 

 this point of view, the history of optics acquires a considerable philo- 

 sophical importance; it becomes the histor}' of the successive progress 

 of our knowledge on the means that nature employs to transmit move- 

 ment and force to a distance. 



The first idea which came to the mind of man (in the savage state) 

 to exercise his force beyond his reach is the throwing of a stone, of an 

 arrow, or of some projectile; this is the germ of the theory of emission. 

 This theory corresponds to a philosophical system which assumes an 

 empty space in which the projectile moves freely. At a more advanced 

 degree of culture, man having become a physicist, has had the more 

 delicate idea of the transmission of movement by waves, suggested at 

 first by the study of waves; afterwards by that of sound. 



This second wa}' supposes, on the other hand, that space is a plenum; 

 there is no longer here transport of matter; particles oscillate in the 

 direction of propagation, and it is by compression or rarefaction of a 

 continuous elastic medium that movement and force are transmitted. 

 Such has been the origin of the theory of luminous waves. Under this 

 form it could onh^ represent a part of the phenomena. It was there- 

 fore insufficient. 



But geometers and physicists before Fresnel did not know of any 

 other undulatorj^ mechanism in a continuous medium. 



The great discovery of Fresnel has been to reveal a third mode of 



