OP RADIANT HEAT THROUGH CRYSTALS. 113 



altered. In this respect also the rays whose plane of polarization 

 coincides with the axis are not to be distinguished from those 

 which pass parallel to the axis. The others however exhibit 

 essential differences. If the two groups of rays remain unse- 

 parated, their total action being alone examined, as in the expe- 

 riments with the natural rays, we must arrive at results which, 

 as regards quantity and quality, lie between those obtained from 

 observations made with the separate polarized groups, where in 

 one instance no difference, and in another the maximum differ- 

 ence is exhibited. The values above quoted furnish abundant 

 proof of this. [J-T.] 



