Physics. 263 
dition in the higher regions of the atmosphere is, I think, certain. 
. At all events, no other assumption than this is necessary to com- 
. pletely account for the firmamental blue and the polarization of 
e sky.* 
e sunbeam would be dive, and it would discharge laterally light 
im precisely the same condition as that discharge the incipient 
ud, In the azure revealed by such a beam would be to 
intents and purposes that which I have called a “blue cloud.”+ 
But, as regards the polarization of the sky, we know that not 
| only is the direction of maximum polarization at right angles to 
i 4 € onta umn : .; 
© «sand could therefore be looked at xery ophenes. without any dis- 
» _ turbance from a solid envelope. At all points of the beam through- 
* Any particles, if small enough, will produce both the color and the polariza- 
tion of the sky. But is the existence of small water-particles on a hot summer’s 
day in the higher regions of our atmosphere inconceivable? It is to be remember- 
ed that the oxygen and nitrogen of the air behave as a vacuum to radiant heat, 
tt 
Practical contact with the cold of space. : ao 
e opinion of Sir John Herschel, connecting the polarization and the blue 
at : : ite 
Color of the x e by the fi “The more the subject [the 
Polarization of align is pain writes eminent philosopher, “the 
More it will und beset with difficulties, and its e: tion when arrived at 
will probably be found to carry with it that of the blue color of sky itself 
and of the great quantity of light it ally doe own tous.” “We ma: 
Pdserve, too,” he adds, “that it is only where the purity of the sky is most abso- 
lute that the ization i i i and that where there 
: t polarization is developed in its highest degree, and that v h 
's the slightest perceptible tendency to cirrus it is materially impaired.” This ap- 
Plies word for word to the “ incipient clouds.” 
