210 PRESNEL. 



rays respectively pass. By making the thickness of 

 such media vary gradually, the rays which traverse them 

 may still destroy or reinforce each other, just as if they 

 had traversed routes perfectly equal. 



It hardly ever happens that any part of space receives 

 direct light alone ; a hundred rays from the same origin 

 arrive at that point after reflections or refractions more 

 or less oblique. Now, after what has been said, we may 



two rays oo> interfere, as in Jig. 1, arriving simultaneously by an 

 equal number of undulations respectively, at u and u>, and thus giv- 

 ing rise to a light stripe at the centre of the screen s, which corre- 

 sponds to the point of concurrence for equal routes, or when the dif- 

 ferences for the different colours are insensible. But now, as in Jig- 2, 

 let the ray o be intercepted by a glass screen G, by which its undula- 

 tions are retarded. When o f has, as before, arrived at /, o will be at 

 M, several undulations behind it: and the point of concurrence of u 

 with M/, will not be the same for different colours, and the central stripe, 

 or point of concurrence for equal equivalent routes, will be that with 

 some after wave u,, or will be at -f- at some distance from c towards 

 G, or the whole body of stripes will be shifted towards the side on 

 which G is placed. 



This was accordingly exactly what Arago found to take place when 

 he placed in the path of the light on one side a transparent screen. 

 The process by which it is effected is most clearly seen by intercept- 

 ing the two rays with two plates of glass of exactly the same thick- 

 ness ; and causing one of them to incline very slightly, so that the ray 

 on that side passes through a slightly greater effective thickness, or is 

 a very little retarded ; the stripes are then seen to shift towards that 

 side, until on increasing the inclination, they disappear altogether. 



So delicate are the indications afforded by this experiment, and so 

 perfect the accordance between the degree of shifting of the fringes, 

 and the refractive power of the intercepting medium, that Arago and 

 Fresnel saw the advantage of employing it for the inverse problem of 

 determining the most minute differences of refractive power, espe- 

 cially those of gases and vapours, for which no other method could 

 be made sufficiently sensible. To demonstrate at once the fact, and 

 the law that this retardation is exactly in proportion to the refractive 

 power of the glass, the translator long ago adopted a simple modifica- 

 tion of this experiment, for an account of which the reader is referred 

 to the Philos. Mag., January, 1832. Translator. 



