to the Wave Theory of Li g] it. 43 



ray. The sum of the distances of the point S from each of the 

 points marked by the letters (PmMpM) that denote (39) the 

 part of the ray contained within the crystal, is proportional to 

 the interval of retardation, that interval being equal to 



' Sm + SM+Sp 



08 



For if from the point E, where the last internal ray M 

 emerges from the second surface of the crystal, a perpendicular 

 El be let fall upon OS, meeting OS in J, the time of describing 

 01 with the velocity V would (43) be 



9 (SP" - Sm" + SM" - Sp" + SM"). 



V x 



But (41) the actual time of describing the broken path PmMpM 

 is 



Tr 9 (PP" + mm" + MM" + pp" + MM") ; 

 V x OS 



and, on inspecting the figure, this time is seen to be greater 

 than the time of describing 01, by 



(SP + Sm + SM+ Sp + SM), 



Fx OS 

 or by the time in which the line 



e 



08 



(SP + Sm + SM+Sp + SM) 



would be described with the velocity V. Consequently, at the 

 moment when the light in the ray SPmMpMS emerges at the 

 point E from the second surface of the crystal, the light in the 

 imaginary uninterrupted ray OS will have passed the point / 

 by an interval equal to the line just mentioned ; and as the two 

 rays afterwards have the same velocity and parallel directions, 

 this interval is the retardation of the emergent ray. 



46. The rays emerging from the first surface after any odd 

 number of internal reflections are to be compared with the 



