342 , FOURTH REPORT — 1834. 



with the increasing ohliquity of the incident pencil, he assumed 

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

 cording to a compUcated law. This assumption is at entire vari- 

 ance 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 progres- 

 sive 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 suhstance 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 refrac- 

 tive indices. It was necessary to suppose, therefore, that in 

 different media, the lengths of the fits varied in the same pi'o- 

 portion ; and, since in the Newtonian theory the refractive in- 

 dices are directly as the velocities of propagation, it followed that 

 as the velocity augmented, the spaces traversed by the ray in 

 the interval of its periodical states, must diminish, and in the 

 same ratio. 



But the facts observet^ by M. Arago and Professor Airy seem 

 to overturn altogether this part of the theory of emission. The 

 rings formed by a plate of air, inclosed between a lens of glass 

 and a metallic reflector, vanish altogether when the light is po- 

 larized perpendicularly to the plane of incidence, and is incident 

 at the polarizing angle of glass. Under these circumstances, 

 no light is reflected from the upper surface of the plate ; but as 

 it is abundantly reflected from the lower, the disappearance of 

 the rings proves that the light reflected from the vpper surface 

 is essential to their production. That the light reflected from 

 the loiver surface also concurs in their formation, appears from 

 the effects observed by M. Arago, when the metallic plate was 

 tarnished ; and we are thus driven to the conclusion that the 

 phenomena arise from the union and mutual influence of the 

 pencils reflected from the two surfaces. 



This mode of explaining the coloiu's of thin plates was pointed 

 o\it by Hooke, in a remarkable passage in his Micrograjjhia, 

 some years before the subject was taken up by Newton. In 

 this passage he very clearly describes the manner in which the 

 rings of successive orders depend on the interval of retardation 

 of the second " pulse," or wave, on the first; and therefore on 

 the thickness of tlie plate. But he does not seem to have had 



