of viewing Newton s Rings. 



251 



yet made any attempt to examine the effect of the re- 

 flexions classed under (d), p. 248. If we look at fig. 8, which 

 is part of fig. 7 on a larger scale, we see that reflexions take 

 place at c, y, z, w, v : now if the path a b c causes the part 

 of the ray m a which takes it to be an odd number of half 

 wave-lengths behind that part of nc which is reflected at c 

 in the direction cd, and thus causes interference in c d ; then 



Fig. 9. 



SLIT 



c y z will cause the ray ab cy z which is reflected at c to be 

 very nearly an even number of half wave-lengths behind the 

 incident ray o z, and therefore this ray will increase the bright- 

 ness of zp: similarly in v q the residual light which reaches v 

 by the path abcy z w v must be an odd number of half wave- 

 lengths behind the light in s v, and will therefore interfere : 

 but since in every case the diminution in intensity caused by 

 these reflexions at a b c y z must be rapid, it is practically only 

 the reflexions a b c y z that can result invisible phenomena. 

 If the region from a to v produces, on the whole, one of the 

 dark rings (using monochromatic light), then if z p, regarded 

 as part of n c y z p and of o z, be the darkest part of the ring, 

 where interference is a maximum, nevertheless at z the part 

 of the ray from a, namely ab cy z, will reinforce the reflected 

 part of the ray o z p, and make the interference less complete 

 than it would otherwise have been ; and, similarly, a bright 

 ring will not be quite so bright as it would be if there were no 

 reflexions in the air-space. In short, the last-considered 

 reflexions superimpose upon the primaries another set not in 

 general coincident with them, and therefore in general 

 weakening their intensity. 



It is also evident, since the more refrangible the light em- 

 ployed, the smaller the diameters of corresponding rings, if we 



