58 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



galaxy ; and lastly, the whole system is alive with 

 movements, the laws of which may one day be recog- 

 nised, though at present they appear too complex to 

 be understood.' 



We can, I think, scarcely claim the establishment of 

 the undulatory theory of light as falling within the 

 last fifty years ; for though Brewster, in his ' Eeport 

 on Optics,' published in our first volume, treats the 

 question as open, and expresses himself still uncon- 

 vinced, he was, I believe, almost alone in his preference 

 for the emission theory. The phenomena of interference, 

 in fact, left hardly any if any room for doubt, and 

 the subject was finally set at rest by Foucault's cele- 

 brated experiments in 1850. According to the undu- 

 latory theory the velocity of light ought to be greater 

 in air than in water, while if the emission theory were 

 correct the reverse would be the case. The velocity 

 of light 186,000 miles in a second is, however, so 

 great that, to determine its rate in air, as compared 

 with that in water, might seem almost hopeless. The 

 velocity in air was, nevertheless, determined by Fizeau 

 in 1849, by means of a rapidly revolving wheel. In 

 the following year Foucault, by means of a revolving 

 mirror, demonstrated that the velocity of light is greater 

 in air than in water thus completing the evidence in 

 favour of the undulatory theory of light. 



The idea is now gaining ground, that, as main- 

 tained by Clerk -Max well, light itself is an electro- 

 magnetic disturbance, the luminiferous ether being the 

 vehicle of both light and electricity. 



Wiinsch, as long ago as 1792, had clearly shown 



