COLOURS OF THIN PLATES. 69 



But the most remarkable of the results obtained by this author 

 relate to the rings formed by the plate of air inclosed between a 

 lens of glass and a metallic reflector. When these were observed 

 in the manner already alluded to, one of the images vanished, as 

 before, at the polarizing angle of glass ; while its appearance, at 

 angles above and below the polarizing angle, presented a remark- 

 able contrast. When the incidence was less than this angle, the 

 two images seen through the double refracting crystal differed 

 only in intensity ; the dimensions and colours of the rings were 

 the same in both. Beyond the polarizing angle, however, the 

 rings in the two images were of complementary colours ; so that if 

 the series in one commenced from a black centre, in the other it 

 began from a white one. The dimensions of the rings of the same 

 order in the two images were also different. Similar phenomena 

 were produced when the thin plate was of a density intermediate 

 to those of the two substances between which it was contained. 

 I shall hereafter have occasion to refer to the observations and 

 deductions of Professor Airy connected with these phenomena. 



When the metallic reflector was slightly tarnished, a second 

 system of rings was visible to the naked eye. The formation of 

 these rings depended on the light irregularly dispersed at. the 

 surface of the metal ; and they were visible, in whatever manner 

 the eye was placed with respect to the incident light. Their tints 

 were complementary to those of the regular series. 



It was soon felt that the phenomena of thin plates were closely 

 connected with some new and fundamental property of light,* 

 and that it was in their application to these phenomena that all 

 theories of light were to be judged. For their explanation, it has 

 been already stated, Newton invented his celebrated doctrine of 

 the " fits of easy reflexion and transmission," a doctrine which will 

 always hold a prominent place in the page of philosophical history. 

 Its application is obvious. The ray is in a fit of easy transmission 

 in its passage through the first surface ; this is succeeded by a fit 

 of easy reflexion, and so alternately. On arriving at the second 

 surface, then, the ray will be in a fit of easy transmission or easy 



* It is unnecessary to refer to the theories of Sir William Herschel, or of M. Par- 

 rot, in both of which the laws of thin plates have been referred to those of reflexion 

 and refraction ; or to that of Mayer, who attempted to reduce them to inflexion. None 

 of these theories have had supporters, and they are all of them inconsistent with 

 oLvious facts. 



