REPORT ON PHYSICAL OPTICS. 34'1 



plate was of a density intermediate to those of the two sub- 

 stances 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 itwasin 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 docti-ine which will 

 always hold a prominent place in the page of philosophical hi- 

 story. 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 reflexion, according as the interval of 

 the surfaces, or the thickness of the plate, is an even or an 

 odd multiple of the length of the fit. Thus the alternate suc- 

 cession of bright and dark rings in homogeneous light, and the 

 arithmetical progression of the thicknesses at which they are 

 exhibited, are satisfactorily explained. To explain the variation 

 in the dimensions of the rings depending on the nature of the 

 light, it is necessary to suppose that the length of the fits 

 varies with the colour, — being greatest in red light, least in 

 violet, and of intermediate magnitude for the rays of interme- 

 diate refrangibility. Newton determined the absolute lengths 

 of these fits for the rays of each simple colour, and found that 

 they bore a remarkable numerical relation to the lengths of 

 the chords sounding the octave. These results are even yet 

 referred to as fundamental data in optical inquiries. 



To account for the remaining laws Newton was constrained to 

 make new suppositions, and to attribute properties to the fits 

 which seem inconsistent with every physical account which has 

 been given of them. Thus, to explain the dilatation of the rings 



• It is unnecessary to refer to the theories of Sir William Herschel or of 

 M. Parrot, in both of wliich 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 obvious facts. 



