88 FRAGMKNTS OF SCIENCE. 



ent refractive indices, and therefore of very different polar- 

 izing angles as ordinarily defined, 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 vvhy^ 

 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 1 

 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." This applied word for word to our 

 " incipient clouds." 



