COLOURS OF THTN PLATES. 143 



reflexion at the first. These two portions, therefore, can 

 never wholly destroy one another by interference, and the 

 intensity of the light in the dark rings can never entirely 

 vanish, as it appears to do when homogeneous light is em- 

 ployed. 



Poisson was the first to point out, and to remedy, this 

 defect in the theory. It is evident, in fact, that there must 

 be an infinite number of partial reflexions within the plate, 

 at each of which a portion is transmitted ; and that it is the 

 sum of all these portions, and not the two first terms of the 

 series only, which is to be considered in the calculation of the 

 effect. When the problem is taken up in this more general 

 form, it is found that, where the effective thickness of the 

 plate is an exact multiple of the length of half a wave, the 

 intensities of the reflected and transmitted lights will be the 

 same as if it were removed altogether, and the bounding 

 media placed in absolute contact. Hence, when these media 

 are of the same refractive power, the reflected light must 

 vanish altogether, and the transmitted light be equal to the 

 incident. 



Here then we have reached a point, with respect to which 

 the two theories are completely opposed. According to both, 

 a certain portion of light is reflected from the first surface of 

 the plate. This portion, in the Newtonian theory, is left in 

 all cases to produce its full effect, and there should there- 

 fore be a considerable quantity of light in the dark rings ; 

 while, in the wave-theory, it is, at certain intervals, wholly 

 destroyed by the interference of the other portions, and the 

 dark rings should be absolutely black in homogeneous light. 



The latter of these conclusions seems to accord with 

 phenomena, while the former is obviously at variance with 

 them. This is clearly shown by an experiment of Fresnel. A 

 prism was laid upon a lens having its lower surface blackened, 

 a portion of the base of the prism being suffered to extend 



