70 REPORT ON PHYSICAL OPTICS. 



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 succession of bright and dark rings in homo- 

 geneous 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 inter- 

 mediate 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 

 with the increasing obliquity of the incident pencil, he assumed 

 that the length of the fits augmented with the incidence, and 

 according to a complicated law. This assumption is at entire 

 variance with the physical theory. If the fits are produced by 

 the vibrations of the ether, which are propagated faster than the 

 rays, and which alternately conspire with and oppose their pro- 

 gressive motion, their lengths should continue the same in the 

 same medium, whatever be the incidence. No attempt, that I am 

 aware of, has been made to reconcile this law with the physical 

 hypothesis of Mr. Melville and M. Biot. 



The same may be said of the variation of the dimensions of 

 the rings with the substance of the reflecting plate. Newton found 

 that when a drop of water was introduced between the glasses, the 

 rings contracted ; and by comparing their diameters in air and in 

 water, he found that the corresponding thicknesses of the plate 

 were as 4 to 3, or in the inverse ratio of the refractive indices. It 

 was necessary to suppose, therefore, that in different media, the 

 lengths of the fits varied in the same proportion ; and, since in the 

 Newtonian theory the refractive indices are directly as the veloci- 

 ties of propagation, it followed that as the velocities augmented, 

 3 spaces traversed by the ray in the interval of its periodical 

 states must diminish, and in the same ratio. 



