ON LIGHT AND COLOURS. 133 



the joint action of the two edges never can account for the 

 difference produced on the fringes, is manifest from hence, 

 that whatever value we give to ?n, we find the proportion of 

 y' : y when x = only that of double, whereas 5 or 6 times 

 is the fact. The same reasoning holds in the case of the 

 spheres of flexion being of different extent: and there are 

 other arguments arising from the analysis on this head, which 

 it would be superfluous to go through, because what is 

 delivered above enables any one to pursue the subject. The 

 demonstration also holds if we suppose the deflective force to 



act as of the distance, while that of inflexion acts as . 

 n m 



But I have taken m = n as simpler, and also as more probably 

 the fact. 



I have said that the rays become less easily inflected and 

 deflected ; but it is plain that on the polarized side they are 

 not inflected or deflected at all. Their disposition on the 

 opposite side is a matter of degree ; their polarization is 

 absolute and their flexion null. 



PROPOSITION III. 



The rays disposed on one side by the first flexion are 

 polarized on that side by the second flexion, and the rays 

 polarized on the other side by the first flexion are depolarized 

 and disposed on that side by the second flexion. 



This proposition is proved by carefully applying the first 

 experiment of Prop. II. ; but great care is required in this 

 experiment, because when three edges are used consecutively, 

 the third edge often appears to act on rays previously acted 

 on by both the other two, when it is only acting on those 

 previously acted on by one or other of those two. Thus when 

 edge A has inflected and edge B aftei'wards deflects the rays 

 disposed by A, a third edge C may, when applied on the side 

 opposite to B, seem to increase the flexion, and yet on re- 

 moving A altogether we may find the same effect continue, 

 which proves that the only action exercised had been by B 

 and C, and that C had not acted on rays previously bent by 



