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. 
jo’ 
