ARTIFICIAL SKY 123 



time after the cessation of perfect polarization, the residual 

 light which passes, when the Nicol is in its position of 

 minimum transmission, is of a gorgeous blue, the whiter 

 light of the cloud being extinguished.* When the cloud 

 texture has become sufficiently coarse to approximate to 

 that of ordinary clouds, the rotation of the Nicol ceases 

 to have any sensible effect on the quantity of light dis- 

 charged normally. 



The perfection of the polarization, in a direction per- 

 pendicular to the illuminating beam, is also illustrated by 

 the following experiment: A Nicol' s prism, large enough 

 to embrace the entire beam of the electric lamp, was placed 

 between the lamp and the experimental tube. A few bub- 

 bles of air, carried through the liquid nitrite of butyl, 

 were introduced into the tube, and they were followed by 

 about three inches (measured by the mercurial gauge) of 

 air which had passed through aqueous hydrochloric acid. 

 Sending the polarized beam through the tube, I placed 

 myself in front of it, my eye being on a level with its 

 axis, my assistant occupying a similar position behind the 

 tube. The short diagonal of the large Nicol was in the 

 first instance vertical, the plane of vibration of the emer- 

 gent beam being therefore also vertical. As the light 

 continued to act, a superb blue cloud, visible to both my 

 assistant and myself, was slowly formed. But this cloud, 

 so deep and rich when looked at from the positions men- 

 tioned, utterly disajjpeared when looked at vertically down- 

 loardj or vertically upward. Reflection from the cloud was 

 not possible in these directions. When the large Nicol 



* This shows that particles too large to polarize the blue, polarize perfectly 

 light of lower refrangibilitj. 



