COLOURS OF THIN PLATES. 



continue the same in the same medium, whatever be the inci- 

 dence. 



The fourth law appears to be also irreconcileable with 

 theory. The thicknesses of the plates of different media, at 

 which the same tint is exhibited, being in the inverse ratio of 

 the refractive indices, it was necessary to suppose that the 

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

 in the Newtonian theory, the refractive indices are directly 

 as the velocities of propagation, it would follow that, as the 

 velocities augmented, the spaces traversed by the ray in the 

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

 proportion. 



(150) Newton seems to have regarded this hypothesis as 

 the mere expression of a physical fact ; and in this light it 

 was long considered. It cannot be denied that, as the thick- 

 ness of the plate increases, the light appears by reflexion 

 and transmission alternately ; and it is of no moment, it may 

 be said, by what name these alternate states are called. 

 But if we look more narrowly into the theory, we shall find 

 that it assumes the alternate appearance of the light, in the 

 reflected and transmitted pencils, to be the effect of an alter- 

 nate reflexion and transmission at a single surface, that surface 

 being the second surface of the plate. Now it can be shown 

 that this supposition is inconsistent with fact ; that light is 

 reflected from both surfaces of the plate ; and that the con- 

 currence of these two reflected pencils is an essential condi- 

 tion of the phenomenon. 



To show this, let us employ (instead of common light) 

 light which is polarized in a plane perpendicular to the plane 

 of incidence ; and let it fall upon a plate of air inclosed be- 

 tween two transparent surfaces of different refractive powers. 

 Under these circumstances it is found that the intensity of 



