THE SKY 149 



phere. Now, the higher we ascend, the more do we leave 

 behind us the particles which scatter the light; the nearer, 

 in fact, do we approach to that vision of celestial space 

 mentioned a moment ago. Viewed, therefore, from the 

 loftiest Alpine summits, the firmamental blue is darker 

 than it is ever observed to be from the plains. 



It is thus shown that by the scattering action of mi- 

 nute particles the blue of the sky can be produced; but 

 there is yet more to be said upon the subject. Let the 

 natural sky be looked at on a fine day through a piece of 

 transparent Iceland spar cut into the form known as a 

 Nicol prism. It may be well to begin by looking through 

 the prism at a snow slope, or a white wall. Turning the 

 prism round its axis, the light coming from these objects 

 does not undergo any sensible change. But when the 

 prism is directed toward the sky the great probability is 

 that, on turning it, variations in the amount of light 

 reaching the eye will be observed. Testing various por- 

 tions of the sky with due diligence, we at length discover 

 one particular direction where the difference of illumina- 

 tion becomes a maximum. Here the Nicol, in one posi- 

 tion, seems to offer no impediment to the passage of the 

 skylight; while, when turned through an arc of ninety 

 degrees from this position, the light is almost entirely 

 quenched. We soon discern that the particular line of 

 vision in which this maximum difference is observed is 

 perpendicular to the direction of the solar rays. The 

 Nicol acts thus upon skylight because that light is polar- 

 ized, while the light from the white wall or the white 

 snow, being unpolarized, is not affected by the rotation 

 of the prism. 



In the case of our manufactured sky not only is the 



