AETIFICIAL SKY. 115 



or fifteen minutes after its first appearance the light 

 from a vividly illuminated actinic cloud, looked at 

 perpendicularly, is absolutely quenched by a Ni col's 

 prism with its longer diagonal vertical. But as the 

 sky-blue is gradually rendered impure by the growth of 

 the particles in other words, as real clouds begin to be 

 formed the polarisation begins to decay, a portion of 

 the light passing through the prism in all its positions. 

 It is worthy of note, that for some time after the cessa- 

 tion of perfect polarisation, 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. 1 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 polarisation, in a direction 

 perpendicular to the illuminating beam, is also illus- 

 trated 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 experi- 

 mental tube. A few bubbles 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 polarised 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 emergent 



1 This shows that particles too large to polarise the blue, polar- 

 \as perfectly light of lower refrangibility. 



