140 FRAGMENTS OF SCIENCE. 



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 com- 

 ing from these objects does not undergo any sensible 

 change. But when the prism is directed towards 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 sky light; 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 polarised, while the light from the 

 white wall or the white snow, being unpolarised, 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 phe- 

 nomena of polarisation are observed even more perfectly 

 than in the natural sky. When the air-space from 

 which our best artificial azure is emitted is examined 

 with the Mcol prism, the blue light is found to be com- 

 pletely polarised at right angles to the illuminating 

 beam. The artificial sky may, in fact, be employed as a 

 second Nicol, between which and a prism held in the 

 hand many of the beautiful chromatic phenomena ob- 

 served in an ordinary polariscope may be reproduced. 



Let us now complete our thesis by following the 

 larger light-waves, which have been able to pass among 

 the aerial particles with comparatively little fractional 

 loss. Without going beyond inferential considerations, 



