Polarization of the Inght reflected by the Sky. 117 
glass; secondly, light passing through the glass endwise, which 
might be recognized by its deep green color; and, thirdly, light 
reflected from the front surfaces of the plates. But all these 
errors would tend to diminish, instead of increase, the polariza- 
tion; and hence, if eliminated, the divergence from theory 
would be still greater. Probably the true explanation is that 
. internal reflection does not take place as completely as theory 
assumes, — y owing to the imperfect transparency of the me- 
partly to the dust and other impurities on the surface. 
It makes but little difference for the reflected rays, the polari- 
zation being the same for three values of 7, namely, 0°, 57°, and 
90°. For the refracted ray, on the other hand, the variations 
are very great, amounting in the case of twenty surfaces, at 90° 
incidence, to over 50 per cent. From the dotted lines we see 
that a partial absence of the internal reflection would account 
for all the results obtained, while neglecting it entirely would 
cause a still greater divergence between theory and observation. 
On account of the thickness of the bundle of ten plates o 
