the Light reflected by the Sky and by Plates of Glass. 139 



effect of sky polarization. Several lines were made with the 

 optical circle and Babinet's wedges, placing the telescopes oppo- 

 site each other, and recording the angles of the Nicol for various 

 positions of the plates. Still a third method was employed, 

 already described in connexion with the Arago's polarimeter. 



It will be noticed that no observations are given of the polar- 

 ization of a beam transmitted by one surface of glass. There 

 seemed to be no easy method of measuring this quantity. It 

 might be done by making a series of prisms of such angles that 

 when the light was incident on one face at 10°, 20°, 30°, &c, the 

 refracted ray would strike normally on the second face. The effect 

 of the latter would then be nothing ; so that the polarization 

 would in this case be entirely due to the first surface. 



To determine the polarization of a single surface of glass, a 

 plate was covered on one side with lampblack ; but it was found 

 to give the same results as when laid on velvet. A plate of 

 coloured glass was therefore used instead. 



Fig. 6 gives the results of the observations of the polariza- 

 tion effected by one surface of glass, fig. 7 that of two surfaces, 

 fig. 8 of eight, fig. 9 the reflected beam for twenty surfaces, and 

 fig. 10 the corresponding refracted beam. The smooth curves 

 give the theoretical polarization, including the internal reflec- 

 tion, and the dotted lines omitting it. The observations of the 

 reflected beam agree very well with theory, especially when the 

 number of surfaces is small. The refracted beams, on the other 

 hand, show a deviation from theory which becomes perceptible 

 for one plate above 80°, for four plates above 65°, and for ten 

 plates above 20°. 



The errors most likely to occur, which would be common to 

 all the observations on the refracted beams, are, first, stray light, 

 or light entering the instrument without passing through the 

 glass ; secondly, light passing through the glass endwise, which 

 might be recognized by its deep green colour; and thirdly, 

 light reflected from the front surfaces of the plates. But all 

 these errors would tend to diminish instead of increase the po- 

 larization ; 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, partly owing to the imperfect transparency of the 

 medium, and partly to the dust and other impurities on the 

 surface. It makes but little difference for the reflected rays, the 

 polarization being the same for three values of i, namely, 0°, 

 57°, and 90°. For the refracted ray, on the other hand, the varia- 

 tions 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 



