RETARDATION. 



209 



has proved by incontestable experiments) the thickness 

 and the refractive power of the body through which the 



ences of route of the concurring [rays become necessarily greater. 

 But white light is a compound of primary coloured rays of different 

 wave lengths. Hence all the interference stripes, except the exactly 

 central ones, are formed by the concurrence of rays having gone 

 through more or less different lengths of route, and consequently with 

 a want of exact concurrence for the different primary rays, which 

 will be greater, as we recede more from the central point; in other 

 words, the stripes towards each side become more and more coloured, 

 and superimposed, till beyond certain limits the stripes disappear, 

 and the whole mixed light is sensibly white. 



Now, if owing to any cause one of the two interfering rays were 

 retarded in its course behind the other, the two rays would not con- 

 cur under the same conditions of equal route, as before, at the central 

 point, but it would not be until at some distance towards the side on 

 which the retardation took place, that they would be, as it were, 

 placed on equal terms to make up for the retardation in the one by 

 greater length of route in the other; the central point of the stripes, 

 and therefore the whole system with it, would thus be shifted towards 

 that side. This may be more clearly illustrated as follows: Let 



Fig. 1. 



Fig. 2. 



c 



