to the Wave Theory of Light. 43 



*ay. The sum of the distances of the point 8 from each of the 

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

 )art of the ray contained within the crystal, is proportional to 

 he interval of retardation, that interval being equal to 



-- (SP + 8m + SM + 8p + SM). 

 G/o 



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

 smerges from the second surface of the crystal, a perpendicular 

 El be let fall upon 08, meeting 08 in /, the time of describing 

 11 with the velocity V would (43) be 



(SP" - Sm" + SM" - 8p" + SM"}. 



3ut (41) the actual time of describing the broken path PmMpM 



s 







OS 



(PP' + mm" + MM" + pp" + MM") ; 



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

 han the time of describing O/, by 



(SP + Sm + SM + Sp + 8M), 



Fx 08 



r by the time in which the line 



-^ (8P + Sm + SM + Sp + SM) 



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

 loment when the light in the ray SPmMpMS emerges at the 

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

 aiaginary uninterrupted ray OS will have passed the point / 

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

 ays afterwards have the same velocity and parallel directions, 

 tiis interval is the retardation of the emergent ray. 



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

 umber of internal reflections are to be compared with the 



