NATURE 733 



lines. Starting with clearly enunciated hypotheses, they deduced 

 from them all their consequences with mathematical rigor, and then 

 compared them with experiment. It seemed to be their aim to give to 

 each of the branches of physics the same precision as to celestial 

 mechanics. 



A mind accustomed to admire such models is not easily satisfied 

 with a theory. Not only will it not tolerate the least appearance of 

 contradiction, but it will expect the different parts to be logically 

 connected with one another, and will require the number of hypo- 

 theses to be reduced to a minimum. 



This is not all; there will be other demands which appear to me 

 to be less reasonable. Behind the matter of which our senses are 

 aware, and which is made known to us by experiment, such a thinker 

 will expect to see another kind of matter the only true matter in 

 his opinion which will no longer have anything but purely geo- 

 metrical qualities, and the atoms of which will be mathematical points 

 subject to the laws of dynamics alone. And yet he will try to repre- 

 sent to himself, by an unconscious contradiction, these invisible and 

 colorless atoms, and therefore to bring them as close as possible to 

 ordinary matter. 



Then only will he be thoroughly satisfied, and he will then imagine 

 that he has penetrated the secret of the universe. Even if the satis- 

 faction is fallacious, it is none the less difficult to give it up. Thus, on 

 opening the pages of Maxwell, a Frenchman expects to find a theo- 

 retical whole, as logical and as precise as the physical optics that is 

 founded on the hypothesis of the ether. He is thus preparing for 

 himself a disappointment which I should like the reader to avoid; 

 so I will warn him at once of what he will find and what he will not 

 find in Maxwell. 



Maxwell does not give a mechanical explanation of electricity and 

 magnetism; he confines himself to showing that such an explanation 

 is possible. He shows that the phenomena of optics are only a par- 

 ticular case of electro-magnetic phenomena. From the whole theory 

 of electricity a theory of light can be immediately deduced. Unfor- 

 tunately the converse is not true; it is not always easy to find a 

 complete explanation of electrical phenomena. In particular it is 

 not easy if we take as our starting-point Fresnel's theory; to do so, 

 no doubt, would be impossible; but none the less we must ask our- 

 selves if we are compelled to surrender admirable results which we 

 thought we had definitively acquired. That seems a step backwards, 

 and many sound intellects will not willingly allow of this. 



Should the reader consent to set some bounds to his hopes, he will 

 still come across other difficulties. The English scientist does not 

 try to erect a unique, definitive, and well-arranged building; he seems 

 to raise rather a large number of provisional and independent con- 



