732 SCIENCE AND HYPOTHESIS 



enunciation of which, without the aid of mathematics, we should be 

 unable to effect. Whether the ether exists or not matters little 

 let us leave that to the metaphysicians; what is essential for us is, 

 that everything happens as if it existed, and that this hypothesis 

 is found to be suitable for the explanation of phenomena. After all, 

 have we any other reason for believing in the existence of material 

 objects? That, too, is only a convenient hypothesis; only, it will 

 never cease to be so, while some day, no doubt, the ether will be 

 thrown aside as useless. 



But at the present moment the laws of optics, and the equations 

 which translate them into the language of analysis, hold good at 

 least as a first approximation. It will therefore be always useful to 

 study a theory which brings these equations into connection. 



The undulatory theory is based on a molecular hypothesis; this is 

 an advantage to those who think they can discover the cause under 

 the law. But others find in it a reason for distrust; and this distrust 

 seems to me as unfounded as the illusions of the former. These 

 hypotheses play but a secondary role. They may be sacrificed, and 

 the sole reason why this is not generally done is, that it would involve 

 a certain loss of lucidity in the explanation. In fact, if we look at it 

 a little closer we shall see that we borrow from molecular hypotheses 

 but two things the principle of the conservation of energy, and 

 the linear form of the equations, which is the general law of small 

 movements as of all small variations. This explains why most of 

 the conclusions of Fresnel remain unchanged when we adopt the 

 electro-magnetic theory of light. 



Maxwell's Theory. We all know that it was Maxwell who con- 

 nected by a slender tie two branches of physics optics and electricity 

 until then unsuspected of having anything in common. Thus 

 blended in a larger aggregate, in a higher harmony, Fresnel's theory 

 of optics did not perish. Parts of it are yet alive, and their mutual 

 relations are still the same. Only, the language which we use to 

 express them has changed; and, on the other hand, Maxwell has re- 

 vealed to us other relations, hitherto unsuspected, between the differ- 

 ent branches of optics and the domain of electricity. 



The first time a French reader opens Maxwell's book, his admira- 

 tion is tempered with a feeling of uneasiness, and often of distrust. 



It is only after prolonged study, and at the cost of much effort, that 

 this feeling disappears. Some minds of high calibre never lose this 

 feeling. Why is it so difficult for the ideas of this English scientist 

 to become acclimatized among us? No doubt the education received 

 by most enlightened Frenchmen predisposes them to appreciate pre- 

 cision and logic more than any other qualities. In this respect the 

 old theories of mathematical physics gave us complete satisfaction. 

 All our masters, from Laplace to Cauchy, proceeded along the same 



