1920] on The Blue Sky and the Optical Properties of Air 153 



to the eye, but this direct method unfortunately does not lend itself 

 to public demonstration. 



We may, however, use polarised light to begin with, and you can 

 then observe that if the polarising Xicol is set so as to transmit up 

 and down vibrations, these are abundantly scattered towards you by 

 the small particles. As I turn the polarising Xicol through a right 

 angle, you will see that the light scattered towards you is extinguished. 



The polarisation of light scattered by the sulphur particles is one 

 of the most conclusive reasons for considering it to be an analogue of 

 the blue light of the sky, for the latter shows a polarisation of exactly 

 the same kind when examined at right angles to the sun. 



A cloud of small particles of any kind is capable of producing 

 these effects, the essential condition being that the individual particles 

 should be of small dimensions compared with the wave-length of 

 light, so that at a given moment the vibration at a given particle may 

 be regarded as having a definite phase. In this case it was shown by 

 my father that the shorter (blue; waves are of necessity more 

 scattered than the longer ones (red) ; thus the scattered light is bluer 

 than the original. This conclusion can be justified in detail whether 

 we adopt the elastic solid theory, or the electro-magnetic theory of the 

 nature of light, but it is also deducible from the general theory of 

 dimensions, without entering upon any details of the nature of light 

 beyond its characterisation by the wave-length. 



An alternative theory which still sometimes shows its head, 

 attributes the colour of the sky to a blueness of the air, regarded as 

 an absorptive medium. Such blueness is referred to the presence of 

 ozone, and appeal is made to the undoubted fact that a sufficiently 

 thick layer of ozone shows a blue colour by absorption. This theory 

 gives no accouut of why the sky light is polarised, or indeed of why 

 there is any light in the clear sky at all. Further, its fundamental 

 postulate that the air is blue by transmission is contrary to observa- 

 tion. The setting sun is seen through a greater thickness of air than 

 the midday sun. According to the theory under discussion, the 

 setting sun ought to be the bluer of the two, which everyone knows 

 it is not. I^o doubt the presence of ozone tends to make the air blue 

 by transmission. But this effect is more than compensated by the 

 lateral leakage (scattering) of blue light from the beam, which makes 

 the transmitted light yellow. 



Dusty Air and Pure Air. 



If it be conceded that the blue sky is due to scattering by small 

 particles, we are confronted with the question of what nature are these 

 particles ? At the time of my father's early investigations (1871) 

 this was left open, though they were regarded as extraneous to the 

 air itself. In 1899 he returned to the subject, and considered the 

 matter from the point of view of what was lost by the original beam 



