484 
Mr Filon, On the variation with the wave-length 
This is obviously 
G cos 2 6 sec yjr 
Mj..- Y t -f Ft 
t 2 1 1 T 1 ^ Akz 2 
A*, 
F a , F 2 being the y 1 and y 2 of the mid-points of the intercepts PQ X , 
Q2Q3 respectively. 
Denote 
and write 
M 1 t 1 , M 2 t 2 
A 
^ = 7/ + 
f 2 =f/- 
AA 2 
ec 2 
by c 2 
c 1 + c 2 > 
c x + c 2 ’ 
so that F/, F 2 ' are now measured from the same base KL (Fig. 4) 1 . 
Then the relative retardation 
= C cos 2 0 sec yfr [ Cl F/ + c 2 F 2 ']. 
Now if y = height of the point Q' above the same base, KL 
and y 0 = height of P above this base, 
T/-2/' = (2/„-2/') (l + Ja) 
v , , , /v , 2 T t 4 - 2 yh + t 2 \ 
U-y=(y.-y)(l + -^ — j. 
Accordingly 
Ci F/ -f- c 2 F 2 ' = 2/0 ( c x + c 2 + 
CiTj + c 2 (2t x + 2^6 + t 2 )' 
y 
2 ya 
/ /CiTj -I- c 2 (2 tj + 2yh 4- t 2 ) 
2 pa 
where 
= PVo~qy say, 
. . CjTj + c 2 (2 tj + 2/x6 + t 2 ) 
^ =C * +C2+ ^ 
C\T X -t- c 2 (2 tj + 2/x6 - 1 - t 2 ) 
Thus 
* 2yaa 
r — C cos 2 0 sec yjr [ py 0 — qy ']. 
Now, in the experiments, both yjr and 6 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 F 2 + e in the text. 
