485 
of the double refraction in strained glass. 
length of the spectrum examined was about 4J cms., so that tan 0 
never exceeded 
J = '04 about. 
2 56 
According^ cos' 2 # at most differed from unity by a quantity of 
the order '0016 which was quite negligible, seeing the accuracy of 
the final measurements could certainly not be greater than \ per 
cent. 
yjr was a quantity of the same order as 0 and the effect of the 
term in yfr will have been to reduce the preceding error by nearly 
one- half, so that the total error introduced by putting the factor 
cos 2 # sec yjr — 1 will be only about 1 in 1000. This we may cer- 
tainly neglect. 
If we do so 
r = G(py„-qy'). 
Now the intensity due to a very thin pencil, whose cross-section 
at Q is dx'dy', will be 
Idx dy' sin 2 27 si n 2 7rr / \, 
I being a constant and 7 being the angle between the axes of the 
polarizer and analyser and the axes in the glass. 
Hence the intensity due to the whole of the light from P 
which passes through the diaphragm 
= / sin 2 27 J dx dy' sinVr/X, 
the integral being taken over the area of the diaphragm. 
This gives : Intensity 
t • 20 [dx'dyf 2irC , 2rrCq , , , 
= / sm 2 27 j — _cos— r (pyo-qv)ws — ^ ^ 
- sin — ( py 0 - qrj ) sin (y - v j , 
f being the height of the centre of the diaphragm above KL. 
The diaphragm being circular, the sine-integral vanishes and 
the others give 
I sin 2 27 - cos (py 0 - qrf) ^ (a)) . 
, 27rCqp 
where a — — , 
\ 
p is the radius of the diaphragm and J x is the Bessel’s function of 
order unity. 
Now if ol < it x T22, J x (a) > 0. 
