COLOURS OF THIN PLATES. 137 



the light in the rings varies with the incidence; and that 

 the whole system disappears in two cases, namely, when the 

 incidence corresponds to the polarizing angle of either of the 

 media. 



To understand the conclusion to which this leads, we 

 must assume a property of light which will be hereafter 

 established namely, that when light, thus polarized, is in- 

 cident upon a transparent surface at what is called the 

 polarizing angle, it is wholly transmitted, and no portion of 

 it whatever reflected. We see then, from the experiment, 

 that the rings disappear when the light reflected from either 

 of the two surfaces of the plate vanishes ; and we are there- 

 fore warranted in concluding, that the light reflected from 

 both surfaces is essential to their production. 



(151) The preceding experiment, and the conclusion 

 drawn from it, lead us to the very threshold of the true 

 theory. In fact, the light incident on the first surface of the 

 plate is in part reflected, and in part also transmitted. The 

 transmitted portion undergoes a similar subdivision at the 

 second surface ; and part of the light reflected at that surface 

 will emerge through the first, and reach the eye along with 

 that reflected there. Thus the reflected light consists of two 

 portions, one reflected at the upper, and the other at the 

 lower surface of the plate ; and these two portions will 

 interfere, and reinforce or weaken each other's effects, ac- 

 cording as they reach the eye in the same or in opposite 

 phases. 



This mode of explaining the phenomena of thin plates 

 was pointed out by Hooke, in a remarkable passage in 

 his Micrographia, some years before the subject was taken 

 up by Newton. In this passage he very clearly describes 

 the manner in which the rings of successive orders depend 



