from Bradley to Fremel. 107 



series are so situated as to correspond to the depressions of the 

 other, they must exactly fill up those depressions, and the 

 surface of the water must remain smooth. Now I maintain 

 that similar effects take place whenever two portions of light 

 are thus mixed ; and this I call the general law of the interference 

 of light." 



Thus, " whenever two portions of the same light arrive to the 

 eye by different routes, either exactly or very nearly in the same 

 direction, the light becomes most intense when the difference of 

 the routes is any multiple of a certain length, and least intense 

 in the intermediate state of the interfering portions ; and this 

 length is different for light of different colours." 



Young's explanation of the colours of thin plates as seen by 

 reflexion was, then, that the incident light gives rise to two 

 beams which reach the eye : one of these beams has been 

 reflected at the first surface of the plate, and the other at the 

 second surface ; and these two beams produce the colours by 

 their interference. 



One difficulty encountered in reconciling this theory with 

 observation arose from the fact that the central spot in Newton's 

 rings (where the thickness of the thin Him of air is zero) is 

 black and not white, as it would be if the interfering beams were 

 similar to each other in all respects. To account for this Young ' 

 showed, by analogy with the impact of elastic bodies, that when -> 

 light is reflected at the surface of a denser medium, its phase 

 is retarded by half an undulation : so that the interfering 

 beams at the centre of Newton's rings destroy each other. The 

 correctness of this assumption he verified by substituting essence 

 of sassafras (whose refractive index is intermediate between those 

 of crown and flint glass) for air in the space between the lenses ; 

 as he anticipated, the centre of the ring-system was now white. 



Newton had long before observed that the rings are smaller ~* 

 when the medium producing them is optically more dense. 

 Interpreted by Young's theory, this definitely proved that the 

 wave-length of light is shorter in dense media, and therefore ^ 

 that its velocity is less. 



