106 The JLuminiferous Medium, 



suppose all refracting media to retain, by their attraction, a, 

 greater or less quantity of the luminous ether, so as to make its- 

 density greater than that which it possesses in a vacuum, 

 without increasing its elasticity." This is precisely the 

 hypothesis adopted later by Fresnel and Green. 



In 1801 Young made a discovery of the first magnitude* 

 when attempting to explain Newton's rings on the principles of 

 the wave-theory. Eejecting Euler's hypothesis of induced 

 vibrations, he assumed that the colours observed all exist in 

 the incident light, and showed that they could be derived from 

 it by a process which was now for the first time recognized 

 in optical science. 



The idea of this process was not altogether new, for it had 

 been used by Newton in his theory of the tides. " It may 

 happen," he wrote, f " that the tide may be propagated from the 

 ocean through different channels towards the same port, and 

 may pass in less time through some channels than through 

 others, in which case the same generating tide, being thus 

 divided into two or more succeeding one another, may produce 

 by composition new types of tide." Newton applied this- 

 principle to explain the anomalous tides at Batsha in Tonkin, 

 which had previously been described by Halley.J 



Young's own illustration of the principle is evidently 

 suggested by Newton's. " Suppose," he says, " a number of 

 equal waves of water to move upon the surface of a stagnant 

 lake, with a certain constant velocity, and to enter a narrow 

 channel leading out of the lake ; suppose then another similar 

 cause to have excited another equal series of waves, which 

 arrive at the same channel, with the same velocity, and at the 

 same time with the first. Neither series of waves will destroy 

 the other, but their effects will be combined ; if they enter the 

 channel in such a manner that the elevations of one series 

 coincide with those of the other, they must together produce a 

 series of greater joint elevations ; but if the elevations of one 



* Phil. Trans., 1802, pp. 12, 387. t Principia, Book in, Prop. 24. 



% Phil. Trans, xiv (1684), p. 681. Young's Works, i, p. 202. 



