OF ifADIANT HEAT THROUGH CRYSTALS. 105 



what may be attributed to errors of observation. It remained 

 thus doubtful whether the calorific rays which penetrate the 

 rock-crystal parallel and perpendicular to the axis are to be re- 

 garded as qualitatively alike, or whether their differences had 

 not merely escaped observation. 



For the further examination of this point, I made use of heat 

 similarly polarized to that which in the former experiments ex- 

 hibited the greatest quantitative differences. 



The Nichol's prism was set in the path of the rays with its 

 principal section horizontal, and the heat, thus polarized in a 

 vertical plane, was permitted to traverse the crystal. Having 

 observed the deflection produced under these conditions, one of 

 the glass plates already made use of was introduced, as before, 

 between the rock-crystal and the thermic pile, and the new de- 

 flection was observed. The question to be decided was, whether 

 the heat, under these circumstances, would exhibit any essential 

 differences as to its capability of transmission through the 

 before-named diathermanous substances, according as the line of 

 transmission through the crystal was parallel or perpendicular 

 to its axis. The following table exhibits the results of the ob- 

 servations. Here, as in the former instances, the ratio of the 

 quantity of heat falling on the diathermanous body to the quan- 

 tity transmitted through it, is given ; the former being expressed 

 by 100. Every number is the result of about thirteen observations. 



Here, comparing the first series of ratios with the second, 

 differences are observed which lie without the limits of error due 

 to observation. Hence calorific rays which have passed through 

 the brown rock-crystal in different directions possess, in different 

 degrees, the power of penetrating one and the same diathermanous 

 body. 



Thus, for example, the heat which has passed through the 

 rock-crystal perpendicular to the axis, passes in greater quantity 

 through the green glass than that which has traversed the crystal 



