88 FRAGMENTS OF SCIENCE. 
ent refractive indices, and therefore of very different polar- 
izing angles as ordinarily denned, but the polarization of 
the beam, by the incipient cloud, has thus far proved itself 
to be absolutely independent of the polarizing angle. The 
law of Brewster does not apply to matter in this condition, 
and it rests with the undulatory theory to explain why. 
Whenever the precipitated particles are sufficiently fine, 
no matter what the substance forming the particles may 
be, the direction of maximum polarization is at right 
angles to the illuminating beam, the polarizing angle for 
matter in this condition being invariably 45 degrees. 
Suppose our atmosphere surrounded by an envelope 
impervious to light, but with an aperture on the sunward 
side through which a parallel beam of solar light could enter 
and traverse the atmosphere. Surrounded by air not directly 
illuminated, the track of such a beam would resemble that 
of the parallel beam of the electric lamp through an incipient 
cloud. The sunbeam would be blue, and it would dis- 
charge laterally light in precisely the same condition as 
that discharged by the incipient cloud. In fact, azure 
revealed by such a beam would be "to all intents and 
purposes " that which I have called a "blue cloud." Con- 
versely our " blue cloud" is, to all intents and purposes, an 
artificial sky.* 
But, as regards the polarization of the sky, we know 
that not only is the direction of maximum polarization at 
right angles to the track of the solar beams, but that at cer- 
tain angular distances, probably variable ones, from the 
sun, " neutral points/' or points of no polarization, exist, on 
both sides of which the planes of atmospheric polarization 
are at right angles to each other. I have made various 
observations upon this subject which are reserved for the 
present; but, pending the more complete examination of 
* The opinion of Sir John Herschel connecting the polarizations 
and the blue color of the sky, is verified by the foregoing results. 
" The more the subject [the polarization of skylight] is considered," 
writes this eminent philosopher, " the more it will be found beset 
with difficulties, and its explanation when arrived at will probably be 
found to carry with it that of the blue color of the sky itself, and of 
the great quantity of light it actually does send down to us." " We 
may observe, too," he adds, " that it is only where the purity of the 
sky is most absolute that the polarization is developed in its highest 
degree, and that where there is the slightest perceptible tendency to 
cirrus it is materially impaired," TU8 applies word, for word to 'our 
"incipient clouds," 
