330 CLASSICS OF MODERN SCIENCE 

 formula due to Lord Kelvin. That calculation leads to figures agree- 

 able to the theory of Maxwell ; but the last doubts will only be dissi- 

 pated when the velocity of propagation has been directly measured. 

 (See Note I.) 



But this is not all. Matters are far from being as simple as this 

 brief account of the matter would lead one to think. There are vari- 

 ous complications. 



In the first place, there is around the exciter a true radiation of 

 induction. The energy of the apparatus radiates abroad, and if no 

 source feeds it, it quickly dissipates itself and the oscillations are 

 rapidly extinguished. Hence arises the phenomenon of multiple res- 

 onance, discovered by Messrs. Sarasin and De la Rive, which at first 

 seemed irreconcilable with the theory. 



On the other hand, we know that light does not exactly follow the 

 laws of geometrical optics, and the discrepancy, due to diffraction, 

 increases proportionately to the wave length. With the great waves 

 of the Hertzian undulations these phenomena must assume enormous 

 importance and derange everything. It is doubtless fortunate, for 

 the moment at least, that our means of observation are as coarse as 

 they are, for otherwise the simplicity which struck us would give 

 place to a dedalian complexity in which we should lose our way. No 

 doubt a good many perplexing anomalies have been due to this. For 

 the same reason the experiments to prove a refraction of the electrical 

 waves can hardly be considered as demonstrative. 



It remains to speak of a difficulty still more grave, though doubt- 

 less not insurmountable. According to Maxwell, the coefficient of 

 electrostatic induction of a transparent body ought to be equal to the 

 square of its index of refraction. Now this is not so. The few 

 bodies which follow Maxwell's law are exceptions. The phenomena 

 are plainly far more complex than was at first thought. But we have 

 not yet been able to make out how matters stand, and the experiments 

 conflict with one another. 



Much, then, remains to be done. The identity of light with a vibra- 

 tory motion in electricity is henceforth something more than a seduc- 

 tive hypothesis ; it is a probable truth. But it is not yet quite proved. 



Note I. — Since the above was written another great step has been 

 taken. M. Blondlot has virtually succeeded, by ingenious experi- 

 mental contrivances, in directly measuring the velocity of a disturbance 



