448 
MR. R. T. GLAZEBROOK ON DOUBLE REFRACTION 
The violet rays, however, diverging as they do from a point on C S, will be incident 
on A P at various angles, most of which, however, will be less than C P N. By 
assuming then the violet rays to issue parallel from the object glass, we have made 
the angle of incidence for the violet too great. 
Again, il Q It be any emergent ray, we have assumed the deviation to be measured 
by D QR 
In the case of the violet rays this again will be too great, and too great by the same 
amount as the angle of incidence. 
We must therefore consider the effect of decreasing the angle of incidence and the 
deviation by the same amount. 
Il (f> be the angle of incidence, \p the angle of emergence, 
xp—l)-\-i—<f> 
8xp=SD — &<f>=0 
Sxp' 8(f)'= 0 
S(p' is negative since S<£ is so 
.'. Sxp' is positive. 
Hence xp is unchanged, xp' is increased. 
The value of /x will therefore in all cases be decreased. 
Now the experimental values of /x are already too great. Hence this alteration will 
tend to bring them more nearly into agreement with theory. The amount of error 
introduced depends on the angle of incidence. To find a general expression for it 
would be a work of difficulty owing to the complicated nature of the formulae involved. 
Let us therefore consider the effect of decreasing the angle of incidence and the 
deviation by 1', (a) near minimum deviation, (b) near grazing incidence for prism I. 
The effects will be much the same for all the prisms. 
We have from Table I. 
0 
00 
o 
to 
36° 8' 
T) + i 
89° 25' 5" 
76° 4' 
0' 
35° 57' 5" 
20° 59' 21" 
1-65367 
1-64623 
By decreasing </> and D + i each by 1' we have the following values:— 
0' 
35° 57' 3" 
20° 59' 5" 
/“ 
1-65355 
1-64591 
