[ 16 ] 



III. On the Relation of the Transmission of Radiant Heat 

 through Cri/stals to the direction of Transmission. By H 

 Knoblauch, Professor of Natural Philosophy in the Univer- 

 sity of Halle*. 



IN a memoir, a full translation of which is given in the last 

 volume of the Scientific IMemoirs, M. Knoblauch showed 

 that radiant heat passed in diffei-ent quantities in different direc- 

 tions through certain uuiaxal crystals ; and also that the quality 

 of the heat, after having traversed the crystals in different direc- 

 tions, was different. In the present paper he extends his re- 

 searches to biaxal crystals. 



The transmission of radiant heat through a number of plates of 

 biaxal crystals, which were all cut perpendicular to a line bisect- 

 ing the acute angle enclosed by the optic axes (the " middle 

 line"), was first examined. A beam of light was conducted by 

 the mirror of a heliostat into a darkened room, and permitted 

 to pass through a Nicholas prism before reaching the crystal. 



When the principal section passing through the obtuse angle 

 of the prism was vertical, the rays issuing from the prism were 

 polarized in a horizontal plane ; and when it was required to 

 polarize them in a vertical plane, the prism was simply turned 

 through an augle of 90 degrees. Having traversed the crystal, 

 the rays fell upon a thermo-electric pile, and the deflection 

 produced was a measure of the amount of heat transmitted. 



Operating in this manner with yelloiu heavy sjjar, M. Knob- 

 lauch found that when the plane of polarization of the rays 

 enclosed an angle of 9Q° with the plane of the optic axes, the 

 heat passed in greater quantity in the direction of the middle 

 line than when both planes were coincident, the ratio of the latter 

 quantity to the former being 100 : 113. 



The quality of the heat which had passed through the crystal 

 was next examined, and proved to be the same, whatever the 

 plane might be in which it was polar-ized. This result was 

 obtained by introducing a diathermanous body in the path of 

 the rays between the crystal and the pile : it was found that the 

 quantity falling on the diathermanous body always bore the same 

 ratio to the quantity transmitted by it, no matter how the rays 

 might have been polarized. 



The next crystal examined was a reddish-yelloiv topaz, and the 

 result was different from that obtained in the case of heavy 

 spar. In topaz the heat travels in greatest quantity in the 

 direction of the middle line, where the plane of polarization 

 coincides with the plane of the optic axes. The ratio of the 

 respective quantities found by M. Knoblauch was as 100 : 96. 



* Abstracted from PoggendoriFs Annalen, vol. xciii. 



