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 sun- 

 beam would be blue, and it would discharge laterally 

 light in precisely the same condition as that discharged 

 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/- Conversely 

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

 artificial sky* 



words, the reflection would require to be made in air upon air ! ' 

 (Sir John Herschel, 4 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 tn the higher regions of 

 our atmosphere inconceivable f It is to be remembered that the 

 oxygen and nitrogen of the air behave as a vacuum to radiant 

 heat, the exceedingly attenuated vapour of the higher atmos- 

 phere being therefore in practical contact with the cold of space. 



* The opinion of Sir John Ilerschel, connecting the polarisa- 



