150 FRAGMENTS OF SCIENCE 



azure of the firmament reproduced, but these phenomena 

 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 Nicol 

 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. 

 Without 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, there- 

 fore, 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 spectrum being more and more withdrawn, the tendency 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 dis- 

 tances, is more and more toward 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 

 getting, or, better still, some time after he has set, leav- 

 ing 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, under favorable atmospheric conditions, 



