294 ON LIGHT. 



in the opposite state or disposition. Having been trans- 

 mitted then at the first, it will be reflected at the second, 

 and, having in its passage back through another equal 

 thickness reassumed its original state in which it first en- 

 tered, it will there be transmitted^ and so will reinforce by 

 its light the general reflected illumination of that surface. 

 Since the per-centage of the total light reflected at any 

 transparent medium is but trifling, the light so sent back 

 from the second surface will be nearly equal to that re- 

 flected from the first. Thus for these exactly interme- 

 diate thicknesses, the joint-reflected illumination is very 

 nearly doubled, and between these and the former series 

 of thicknesses will increase and diminish alternately and 

 gradually. 



(77.) Suppose now in the case of our soap-bubble 

 the thickness of the film to increase uniformly outwards 

 from its vertex (where it is nearly nil). Then it is evi- 

 dent that when exposed to dispersed light it will appear 

 divided into equivalent circular zones alternately bright, 

 and comparatively dark, the centre being also dark. And 

 here we have a representation of our observed rings, with, 

 however, this remarkable and most important difference, 

 viz. : that the central spot and the dark divisions, on this 

 explanation, ought not to appear absolutely black, but 

 half bright, when compared with the brightest portions 

 between them. In point of fact, some exceedingly slight 

 reflexion is perceivable in the dark centre, but instead 

 of half, it cannot be estimated at the fiftieth part of the 

 illumination of the" bright ring which immediately ad- 

 joins it 



