484 Mr Filon, On the variation with the wave-length 



This is obviously 



G cos 2 6 sec yjr 



»• j>. + * Jwr. 



A.& 2 * * ' AJfc, 



11 



Fj, F 2 being the ^ and y 2 of the mid-points of the intercepts PQ U 

 Q2Q3 respectively. 



_ MjTj, , M 2 r 2 . 



Den0te AA 2 y Cl ' A~F J ° 2 



and write Fj = F/ + 



"22 



ec 2 



Ci + c 2 ' 

 eCi 



Ci + c 2 ' 



so that F/, F/ are now measured from the same base KL (Fig. 4) 1 . 

 Then the relative retardation 



= C cos 2 6 sec yfr [cj F/ + c 2 F 2 ']. 



Now if 2/' = height of the point Q' above the same base, KL 

 and y = height of P above this base, 



F/-y = ( y „-y)(l + ^) 



vi 'i '%/Y , 2t, + 2/t6 + t„\ 

 F s -2/=(y.-y)(i + ^ ). 



Accordingly 



_ , / CiT! + c 2 (2t! + 2ya6 + T 2 ) \ 

 y V 2^a J ' 



where y = Ci + c^ + ° 2 ^ + ^ + T>) , 



2yu,a 



Thus 



CiTj j- C 2 (2T! + 2,0,6 + T 2 ) 



q ~ 2fia 



r = G cos 2 sec y\r [ py — qy'\ 



Now, in the experiments, both yjr and were small. The focal 

 length of the condensing lens being about 56 cms., the total 



1 The length marked Y 2 in Fig. 4 is measured from CD as base and is 

 equivalent to Y 2 + e in the text. 



