THE SKY. 141 



particles upon the solar light increases with the atmo- 

 spheric 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 withdrawn, the tendency is to give the longer waves 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 enchanced 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 in- 

 ference, 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 moun- 

 tain heads, under favourable atmospheric conditions, 

 shine like rubies. And all this splendour is evoked by 

 the simple mechanism of minute particles, themselves 

 without colour, suspended in the air. Those who 

 referred the extraordinary succession of atmospherie 

 glows, witnessed some years ago, to a vast and violent 

 discharge of volcanic ashes, were dealing with c a true 

 cause.' The fine floating residue of such ashes would, 

 undoubtedly, be able to produce the effects ascribed to 

 it. Still, the mechanism necessary to produce the 

 morning and the evening red, though of variable effi- 

 ciency, is always present in the atmosphere. I have 

 seen displays, equal in magnificence 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 repetition of the glows which rendered 

 the volcanic theory highly probable. 



