THE SKY. 141 



we can state what must occur. The action of the par- 

 ticles upon the solar light increases with the atmospheric 

 distances traversed by the sun's rays. The lower the 

 sun, therefore, the greater the action. The shorter 

 waves of the spectrum being more and more withdrawn, 

 the tendency is to give the longer waves an enhanced 

 predominance in the transmitted light. The tendency, 

 in other words, of this light, as the rays traverse ever- 

 increasing distances, is more and more towards red. 

 This, I say, might be stated as an inference, but it is 

 borne out in the most impressive manner by facts. 

 When the Alpine sun is setting, or, better still, some 

 time after he has set, leaving the limbs and shoulders of 

 the mountains in shadow, while their snowy crests are 

 bathed by the retreating light, the snow glows with 

 a beauty and solemnity hardly equalled by any other 

 natural phenomenon. So, also, when first illumined 

 by the rays of the unrisen sun, the mountain heads, un- 

 der favourable atmospheric conditions, shine like rubies. 

 And all this splendour is evoked by the simple mech- 

 anism of minute particles, themselves without colour, 

 suspended in the air. Those who referred the extraor- 

 dinary succession of atmospheric glows, witnessed some 

 years ago, to a vast and violent discharge of volcanic 

 ashes, were dealing with ' a true cause/ The fine float- 

 ing residue of such ashes would, undoubtedly, be able 

 to produce the effects ascribed to it. Still, the mechan- 

 ism necessary to produce the morning and the evening 

 red, though of variable efficiency, is always present in 

 the atmosphere. I have seen displays, equal in magnifi- 

 cence to the finest of those above referred to, when 

 there was no special volcanic outburst to which they 

 could be referred. It was the long-continued repeti- 

 tion of the glows which rendered the volcanic theory 

 highly probable. 



