ARTIFICIAL SKY. 117 



substances of widely different refractive indices, and 

 therefore of very different polarising angles as ordi- 

 narily defined, but the polarisation of the beam, by the 

 incipient cloud, has thus far proved itself to be abso- 

 lutely independent of the polarising angle. The law 

 of Brewster does not apply to matter in this condition, 

 and it rests with the undulatory theory to explain why. 

 Whenever the precipitated particles are sufficiently fine, 

 no matter what the substance forming the particles 

 may be, the direction of maximum polarisation is at 

 right angles to the illuminating beam, the polarising 

 angle for matter in this condition being invariably 45. 

 Suppose our atmosphere surrounded by an envelope 

 impervious to light, but with an aperture on the sun- 

 ward side through which a parallel beam of solar light 

 could enter and traverse the atmosphere. Surrounded 

 by air not directly illuminated, the track of such a 

 beam would resemble that of the parallel beam of 

 the electric lamp through an incipient cloud. The 

 sunbeam would be blue, and it would discharge later- 

 ally light in precisely the same condition as that dis- 

 charged by the incipient cloud. In fact, the azure 

 revealed by such a beam would be to all intents and 

 purposes that which I have called a ' blue cloud.' Con- 

 versely our ' blue cloud ' is, to all intents and purposes, 

 an artificial sky. 1 



reflection would require to be made in air upon air 1 ' (Sir John 

 Herschel, ' Meteorology,' par. 233.) 



Any particles, if small enough, will produce both the colour and 

 the polarisation of the sky. But is the existence of small water- 

 particles on a hot summer's day in the higher regions of our atmo- 

 tphere inconceivable ? It is to be remembered that the oxygen and 

 nitrogen of the air behave as a vacuum to radiant heat, the exceed- 

 ingly attenuated vapour of the higher atmosphere being therefore 

 in practical contact with the cold of space. 



1 The opinion of Sir John Herschel, connecting the polarisation 

 and the blue colour of the sky, is verified by the foregoing results. 



