TJ1U71. 



COLOURS OF THIN PLATES. 141 



panied by reflexion ; and this reflexion is greater, the greater 

 the difference of the densities of the ether in the two media. 

 It appears also, from what has been said, that the direction 

 of the motions of the particles of the first medium, after they 

 communicate motion to those of the second, will be different 

 according as the ether is denser or rarer in the first medium. 

 In the former case the vibration of the particles is in the 

 same direction that it was before ; in the latter it is in the 

 opposite direction. Thus there will be a reflected wave in both 

 cases ; but in one case this reflected wave is caused by a 

 vibration in the same direction as that of the incident wave ; 

 in the other, by a vibration in an opposite direction. 



The result of this difference is obviously the same as if 

 one of the systems of waves were to gain or lose half an 

 undulation on the other ; so that when the waves reflected 

 from the two surfaces of the plate should be in complete ac- 

 cordance, as far as depended on the difference of the lengths 

 of their paths, they will actually be in complete discordance, 

 and vice versa. Thus the dark rings will be formed where 

 the thickness of the plate is any even multiple of X sec 0, 

 and the bright ones where that thickness is an odd multiple 

 of the same quantity ; and the facts and the theory are re- 

 conciled. This explanation of the phenomenon was given by 

 Young. 



(154) The principle which we have been illustrating has 

 been experimentally established by M. Babinet, by an inde- 

 pendent method. A pencil of rays diverging from a narrow 

 aperture is separated into two, slightly inclined to one 

 another, by means of the obtuse prism (108). These are 

 allowed to fall on a thick plate of parallel glass, whose second 

 surface is quicksilvered in one-half of its extent ; and in such 

 a manner that both may be reflected by the transparent portion 

 of that surface, or both by the opaque portion, or one by the 



