POLARIZED LIOHT. 121 



of polarized light due to reflection alone. In any case, how- 

 ever, after the reflection at the thin-films, interference and the 

 appearance of thin-film colors would result in the usual way. 

 And, finally, these colored rays in passing through the crystal 

 after reflection might be further changed. It is evident, 

 therefore, that an exact analysis of the action in detail is out 

 of the question here. Perhaps the best we can do is to attri- 

 bute most of the polarization to the polarizing property of the 

 spar and the spectral colors in the main to the interference 

 after reflection at the thin-films. 



Again, a natural source of polarized light not familiar to 

 eveiyone is to be found in the light of the sky viewed at right 

 angles to the rays of the sun. Thus, if the southern sky be 

 looked at through a Nichol prism toward the hour of sunset, 

 the light will l^e found polarized and will exhibit alternate 

 light and darkness as the analyser is rotated. A piece of 

 ordinary mica held. up against the sky and viewed through 

 the Nichol will then exhibit most beautiful colors as it is 

 moved about before the eye. 



The same kind of polarization can be seen by moonlight. 

 If, on a brightly moonlit night, the sky is examined with a 

 Xichol prism in a direction at right angles to the moon's rays, 

 the light will be found very perceptibly polarized. An inter- 

 posed piece of mica will show alternately light on a dark field 

 and dark on a light field, as the prism is rotated. The con- 

 trasts of light and dark are quite strong, but the light is not 

 sufficiently intense to show color. If the sky is looked at in 

 the direction of the moon no polarization is visible. Although 

 the moonlight is produced by reflection, it is not the kind of 

 reflection, nor at the proper angle, to produce much polarized 

 light. 



It is needless to say man}' other natural sources of polar- 

 ized light occurring around us might l)e mentioned. The 

 examples cited, however, may serve the purpose of the writer 

 ami interest some new disciple in this interesting field of study. 



