NATURE 707 



to-day will in a short time succumb in their turn, and he concludes 

 that they are absolutely in vain. This is what he calls the bank- 

 ruptcy of science. 



His skepticism is superficial; he does not take into account the 

 object of scientific theories and the part they play, or he would under- 

 stand that the ruins may be still good for something. No theory 

 seemed established on firmer ground than Fresnel's, which attributed 

 light to the movements of the ether. Then if Maxwell's theory is 

 to-day preferred, does that mean that Fresnel's work was in vain? 

 No; for Fresnel's object was not to know whether there really is an 

 ether, if it is or is not formed of atoms, if these atoms really move in 

 this way or that; his object was to predict optical phenomena. 



This Fresnel's theory enables us to do to-day as well as it did before 

 Maxwell's time. The differential equations are always true, they 

 may be always integrated by the same methods, and the results of this 

 integration still preserve their value. It cannot be said that this is 

 reducing physical theories to simple practical recipes; these equations 

 express relations, and if the equations remain true, it is because the 

 relations preserve their reality. They teach us now, as they did then, 

 that there is such and such a relation between this thing and that; 

 only, the something which we then called motion, we now call electric 

 current. But these are merely names of the images we substituted for 

 the real objects which Nature will hide forever from our eyes. The 

 true relations between these real objects are the only reality we can 

 attain, and the sole condition is that the same relations shall exist be- 

 tween these objects as between the images we are forced to put in 

 their place. If the relations are known to us, what does it matter if 

 we think it convenient to replace one image by another? 



That a given periodic phenomenon (an electric oscillation, for in- 

 stance) is really due to the vibration of a given atom, which, behaving 

 like a pendulum, is really displaced in this manner or that, all this is 

 neither certain nor essential. But that there is between the electric 

 oscillation, the movement of the pendulum, and all periodic phenom- 

 ena an intimate relationship which corresponds to a profound reality; 

 that this relationship, this similarity, or rather this parallelism, is 

 continued in the details; that it is a consequence of more general 

 principles such as that of the conservation of energy, and that of 

 least action; this we may affirm; this is the truth which will ever 

 remain the same in whatever garb we may see fit to clothe it. 



Many theories of dispersion have been proposed. The first were 

 imperfect, and contained but little truth. Then came that of Helm- 

 holtz, and this in its turn was modified in different ways; its author 

 himself conceived another theory, founded on Maxwell's principles. 

 But the remarkable thing is, that all the scientists who followed Helm- 

 holtz obtain the same equations, although their starting-points were 



