On the Transmission of Radiant Heat through Crystals. 17 



Qualitative differences vvere also observed in this case, de- 

 pendent on the position of the plane of polarization. Glasses 

 of different colours were used to test the quality of the heat. 

 Calling the quantity falling upon the pile previous to the intro- 

 duction of the glass 100, the ratio of this quantity to that 

 which passed through the glass in the respective cases is given 

 in the foUowins; table : — 



We must conclude from this table that the heat whose plane 

 of polarization coincides with that of the optic axes, possesses, 

 after its passage through the topaz, a greater power of traversing 

 the blue glass than the heat whose plane of polarization is per- 

 pendicular to that of the optic axes. The former, however, 

 passes through the green glass in less quantity than the latter. 



These differences were exhibited in a far more striking manner 

 in the case of a plate of ijellow-green hornblende. For the sake 

 of brevity, we will call the quantity of heat which passes through 

 the crystal when the planes of the optic axes and that of polari- 

 zation are coincident, Q, and the quantity which passes when 

 the planes are at right angles to each other, Q', In the case of 

 hornblende, the ratio of Q to Q' was as 100 : 133. 



Qualitative differences also exhibited themselves here, as is 

 manifest from the following table : — 



We here observe, that the rays which pass in greatest quantity 

 through the red and yellow glass are the most intercepted by 

 the green and blue. We also see that a characteristic difference 

 exists between the heat issuing from hornblende and that issuing 

 from topaz. For the heat whose plane of polarization coincides 

 with that of the optic axes, possesses, in the case of topaz, a 

 higher power, and in the case of hornblende a lower power, to 

 radiate through the blue glass, than the heat whose plane of 

 polarization crosses that of the optic axes. 



In the case of a ))latc of pistucite the ratio of Q to Q' was 

 found to be as 100 : 14G. 



* i. e. the pluue of polarization and the jjlanc of the optic axes. 



Phil. May. S. 4. Vol. 10. No. 03. July 1855. C 



