INTERFERENCE OF LIGHT. 91 



sary for their interference. It is found, accordingly, that a 

 series of alternate bright and dark bands is formed parallel 

 to the edge of the prism. 



(109) It will be evident, from what has been said, that 

 the central fringe produced by the interference of two pencils 

 is the locus of those points at which they arrive in the same 

 time; and, accordingly, when neither of the pencils has ex- 

 perienced any interruption in its progress, the points of that 

 fringe will be equally distant from the two luminous origins. 

 The case is altered, however, if we interpose a thin plate of a- 

 denser medium in the path of one of the interfering rays. 

 If the light is retarded in the denser medium, it is obvious 

 that the points reached in the same time will no longer be 

 equally distant from the two centres, but will be nearer to- 

 that whose light has undergone the retardation. The reverse 

 will be the case if the light is accelerated in the interposed 

 plate ; so that the central fringe, and the whole system, will 

 be shifted towards the side of the interposed plate in the 

 former case, and from it in the latter. Here then we have a 

 complete experimentum crucis, by which to decide between the 

 theory of emission and that of waves; and its result, as 

 we have already stated, is conclusive in favour of the wave- 

 theory. 



It is easy -to calculate the relation between the thickness 

 and refractive index of the plate, and the displacement of the 

 fringes. Let a and a' denote the distances of the displaced 

 central fringe from the two origins ; and let e be the thick- 

 ness of the plate interposed in the path of the pencil diverg- 

 ing from the first. Then as the times of traversing these two 

 distances are the same, we have 



a-e e a' 

 v v f v ' 



