LIGHT AND ELECTRICITY. 137 



On tlie other liaiid, we know that liglit does not exactly follow the 

 laws of geonietrieal 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 jyerplexing anomalies have been due to this. For the 

 same reason the experimeuts 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 doubtless 

 not insurmountable. According to ^laxwell, the coefticient of electro- 

 static 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 IMaxwelFs law are exceptions. The phenomena are plaiidy far 

 raoie 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 bo done. The identity of light with a vibra- 

 tory motion in electricity is hencefcn-th something more than a seductive 

 hy])othesis; it is a probable truth. lUit it is not yet quite proved. 



Note 1. — Since the above was written another great step has been 

 taken. M. Blondlot has virtually succeeded, by ingenious experimental 

 contrivances, in directly measuring the velocity of a disturbance along 

 a wire. The number found differs little from the ratio of the units; 

 that is, from the velocity of light, which is ;>00,000 kilometers per sec- 

 ond. Since the interference experiments made at Geneva by Messrs. 

 Sarasin and Be la Kive have shown, as I said above, that induction is 

 proi)agated in air with the same velocity as an electric disturbance 

 which follows a conducting wire, we must conclude that the velocity of 

 the induction is the same as that of light, which is a contirmation of 

 the ideas of Maxwell. 



M. Fizeau had formerly found for the velocity of electricity a number 

 far smaller, about 180,000 kilometers. But there is no contradiction. 

 The currents used by ]\I. Fizeau, though intermittent, were of small 

 frequency and penetrated to the axis of the Avire, while the currents of 

 M. Blondlot, oscillatory and of very short period, remained superficial 

 and were confined to a layer of less than a hundredth of a millimeter 

 in thickness. ( )ne may readily suppose the laws of i)ropagation are not 

 t'le same in the two cases. 



Note II. — I have endeavored above to render the explanation of the 

 electrostatic attractions and of the phenomena of induction comprehen- 

 sible by means of a simile. Now let us see what Maxwell's idea is of 

 the cause which produces the mutual attractions of currents. 



' Tliat is, it fs not so when we inalve tlie calculation with the indexes of refraction 

 of tlie waves of visible lijiht. — Translator. 



