STRUCTURE AND LIGIIT OF THE SKY. 267 



not, be able to overcome this difficulty, and to reach to- 

 gether a greater elevation. 



THE SKY OF TEE ALPS. 



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The vision of an object always implies a differential 

 action on the retina of the observer. The object is dis- 

 tinguished from surrounding space by its excess or defect 

 of light in relation to that space. By altering the illumi- 

 nation, either of the object itself or of its environment, we 

 alter the appearance of the object. Take the case of clouds 

 floating in the atmosphere with patches of blue between 

 them. Any thing that changes the illumination of either 

 alters the appearance of both, that appearance depending, 

 as stated, upon differential action. Now the light of the 

 sky being polarized, may, as the reader of the foregoing 

 pages knows, be in great part quenched by a Nicol's 

 prism, while the light of a cloud, being unpolarized, cannot 

 be thus extinguished. Hence the possibility of very re- 

 markable variations, not only in the aspect of the firma- 

 ment, which is really changed, but also in the aspect of the 

 clouds which have that firmament as a background. It is 

 possible, for example, to choose clouds of such a depth of 

 shade that when the Nicol quenches the light behind them, 

 they shall vanish, being undistinguishable from the residual 

 dull tint which outlives the extinction of the brilliance of 

 the sky. A cloud less deeply shaded, but still deep enough, 

 when viewed with the naked eye, to appear dark on a 

 bright ground, is suddenly changed to a white cloud on a 

 dark ground by the quenching of the sky behind it. When 

 a reddish cloud at sunset chances to float in the region of 

 maximum polarization, the quenching of the sky behind it 

 causes it to flash with a brighter crimson. Last Easter eve 

 the Dartmoor sky, which had just been cleansed by a snow- 



