1 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 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. 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being more and more withdrawn, the tendency is to give 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 
