256 FRAGMENTS OF SCIENCE. 



position it is transmitted. Our way is now, to some extent, 

 cleared toward an examination of the light of the sky. 

 Looking at various points of the blue firmament through a 

 Nicol's prism, and turning the prism round its axis, we im- 

 mediately notice variations of brightness. In certain posi- 

 tions of the prism, and from certain points of the firmament, 

 the light appears to be freely transmitted ; while it is only 

 necessary to turn the prism round its axis through an angle 

 of 90 to materially diminish the intensity of the light. On 

 close scrutiny it is found that the difference produced by 

 the rotation of the prism is greatest when the sky is re- 

 garded in a direction at right angles to that of the solar 

 rays through the air. 



Let me describe a few actual observations made some 

 days ago on Primrose Hill. The sun was near setting, and 

 a few scattered neutral-tint clouds, which failed to catch 

 the dying light, were floating in the air. When these were 

 looked at across the track of the solar beams, it was pos- 

 sible, by turning the Nicol round, to see them either as 

 white clouds on a dark ground, or as dark clouds on a bright 

 ground. 1 In certain positions of the prisms the sky-light 

 was in great part quenched, and then the clouds, projected 

 against the darkness of space, appeared white. Turning 

 the Nicol 90 round its axis, the brightness of the sky was 

 restored, the clouds becoming dark through contrast with 

 this brightness. Experiments of this kind prove that the 

 blue light sent to us by the firmament is polarized, and that 

 the direction of most perfect polarization is perpendicular 

 to the solar rays. Were the heavenly azure like the light 

 scattered from a thick cloud, the turning of the prism would 

 have no effect upon it ; it would be transmitted equally dur- 

 ing the entire rotation of the prism. The light of the sky is 

 in great part quenched, because it is in great part polarized. 



1 1 was not aware when these words were written that this observation 

 was made by the indefatigable Brewster. 



