3 06 PR A GMENTS OF SCIENCE, 



of polarization are observed even more perfectly than in 

 the natural sky. When the air-space from which our best 

 artificial azure is emitted is examined with the ISTicol prism, 

 the blue light is found to be completely polarized at right 

 angles to the illuminating beam. The artificial sky may, 

 in fact, be employed as a second Nicol, between which and 

 a prism held in the hand many of the beautiful chromatic 

 phenomena observed in an ordinary polariscope may be 

 reproduced. 



Let us now complete our thesis by following the larger 

 light- waves, which have been able to pass among the aerial 

 particles with comparatively little fractional loss. With- 

 out going beyond inferential considerations, we can state 

 what must occur. The action of the particles 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 being more and more withdrawn, the tendency is to give 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 trans- 

 mitted light. The tendency, in other words, of this light, 

 as the rays traverse ever-increasing distances, is more and 

 more toward red. This, I say, might be stated as an infer- 

 ence, 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 equaled by any other natural phenomenon. 

 So, also, when first illumined by the rays of the unrisen 

 sun, the mountain heads, under favorable atmospheric con- 

 ditions, shine like rubies. And all this splendor is evoked 

 by the simple mechanism of minute particles, themselves 

 without color, suspended in the air. Those who referred 

 the extraordinary succession of atmospheric glows, wit- 

 nessed some years ago, to a vast and violent discharge of 

 volcanic ashes, were dealing with " a true cause." The 

 fine floating 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 atmos- 

 phere. 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 



