THE SKY. 105 



mathematical formula to express the diminution of the 

 blue, in arcs drawn east and west from the zenith down- 

 ward. This diminution is a natural consequence of the 

 predominance of coarser particles in the lower regions of 

 the atmosphere. Were the particles which produce the 

 purer celestial vault all swept away, we should, unless 

 helped by what has been called "cosmic dust," look into 

 the blackness of celestial space. And were the whole 

 atmosphere abolished along with its suspended matter, we 

 should have the " blackness " spangled with steady stars; for 

 the twinkling of the stars is caused by our atmosphere. 

 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 minute 

 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 trans- 

 parent 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 portions of the sky 

 with due diligence, we at length discover one particular 

 direction where the difference of illumination becomes a 

 maximum. Here the Nicol, in one position, 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 sky- 

 light because that light is polarized, while the light from 

 the tfhite 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 

 azure of the firmament reproduced, but these phenomena 



