Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/126

 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. Rejecting 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, : "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.

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: