322 HERMANN VON HELMHOLTZ 



same electro-dynamic action in its neighbourhood as a galvanic 

 current in a conductor. In order to obtain evidence for this, 

 Helmholtz made it the subject of one of the great prize com- 

 petitions of the Academy, by which Hertz was led to his 

 remarkable discoveries. These afforded the direct proof of 

 the accuracy of the hypothesis which Faraday and Maxwell 

 had advanced as highly probable, i.e. that the oscillations of 

 light are electrical oscillations in the ether that occupies space, 

 and that this in itself possesses the properties of an insulator 

 and a magnetizable medium. 



* There can no longer be any question/ says Helmholtz later 

 in his classical preface to Hertz's Principles of Mechanics, 'that 

 the luminous vibrations are electric vibrations in the ether that 

 fills space, and that the latter itself possesses the properties 

 of an insulator and a magnetic medium. The electric oscilla- 

 tions in the ether represent an intermediate stage between the 

 relatively slow motions produced by the elastic and resonant 

 vibrations of a magnetic tuning-fork, and the enormously swift 

 vibrations of light, but it can be shown that their rate of 

 propagation, their character as transverse oscillations, and the 

 concomitant possibility of polarization phenomena, and of 

 refraction and reflection, correspond completely with those 

 obtaining in the case of light and heat rays. The electrical 

 waves are only lacking in capacity to affect the eye, like the 

 dark heat rays, since their frequency is not sufficiently high. 

 It was verily a great achievement to acquire absolute proof 

 that light, that mysterious and powerful force of nature, is so 

 nearly related to a second equally mysterious and perhaps still 

 more potent force, that of electricity. For theoretical science 

 it is perhaps even more important to understand how what 

 appears to be action at a distance can be propagated by trans- 

 lation of action from one layer of the intermediate medium to 

 the next. The mystery of gravitation indeed remains, since 

 we cannot yet logically explain it otherwise than as due to 

 a true action at a distance/ But it took Hertz nearly the 

 whole of the last decade but one of the Nineteenth Century to 

 establish and work out his mighty conception, which is to-day 

 the foundation of the whole modern doctrine of electricity. 



In the Easter holidays Helmholtz visited Ludwig at Leipzig, 

 and wandered through the Schwarzathal in Thuringia, and at the 



