of viewing Newton's Rings, 247 



before, the images are arranged as in fig. 4, where B x is caused 

 by light which, originally reflected from the lower surface of 



Fig. 4. 



1 |A 





i i*. 





1 |A* 





[ IB, 





f |8 ? 





[ IC, 





1 lc ? 



the plate, has undergone a second reflexion from the upper 

 interior surface, and a third from the lower interior surface of 

 the plate. Similarly, the light by which B is seen has under- 

 gone five internal reflexions ; C has undergone seven ; C x nine; 

 and so on. Now when the second plate is added beneath the 

 first it gives rise to a similar series, but more complicated 

 from the fact that the second plate's reflexions are not only 

 caused by its internal surfaces but also by its external upper 

 surface. 



The course of the rays forming the first few reflexions is 

 easily seen from fig. 5. By backing the plate in the usual 



Fig. ba. 



way we con practically suppress the reflexions from the lower 

 internal surface of the lower plate ; and since A t is the sole 

 reflexion from the upper surface of the upper plate, it follows 

 that we see in A 2 , B 1? B 2 , &c, the results of reflexions in (a) 

 the internal upper surface of the upper plate, (b) the internal 



