22 On the Transmission of Radiant Heat through Crystals. 



here it was fouud that the calorific rays passed in greatest 

 quantity parallel to the supplementary line, in less quantity 

 perpendicular to the plane of the optic axes, and least of all in 

 the direction of the middle line, a deportment antithetical to 

 that of dichroite being thus exhibited. Small qualitative differ- 

 ences were also observed where the heat was transmitted through 

 the various coloured glasses. 



Remarkable quantitative and qualitative differences were shown 

 in the case of a twin ciystal of diopside, according as the calorific 

 rays were transmitted through it in one or the other direction. 

 From all the experiments made with biaxal crystals, M. Knoblauch 

 considers the fact to be placed beyond doubt, that certain of 

 these crystals transmit calorific rays with different facilities in 

 different directions ; and after this transmission exhibit different 

 properties; further, that for polarized heat, differences are ob- 

 served in one and the same direction according as the plane 

 of ])olarization coincides with or is perpendicular to that of the 

 optic axes. 



In the memoir alluded to at the commencement of this abs- 

 tract, M. Knoblauch showed that the transmission of heat 

 through certain crystals possessing but one optic axis, was de- 

 pendent on the direction of transmission. In addition to the 

 ol)servations there recorded, he has recently made others on 

 amethyst and idocrase, and finds, in the case of amethyst, when 

 polarized heat is used, that the calorific rays which pass per- 

 pendicular to the axis of the crystal are different, in quantity 

 and quality, according as their plane of polarization occupies 

 the one or the other position ; but when the rays pass in the 

 direction of the crystallographie axis, no such differences are 

 observed. 



The transmission of unpolarized heat was not examined in the 

 case of amethyst, but in the case of idocrase the quantity of 

 natural rays transmitted parallel to the axis was to the quantity 

 transmitted perpendicular to the axis as 100 : 78; qualitative 

 differences were also exhibited by the two groups of rays. 



These differences were not observed in the rays which passed 

 through the crystal in two directions at right angles to each 

 other, and also at right angles to the optic axis. 



When polarized heat was used, the quantities transmitted 

 when the plane of polarization was respectively parallel and 

 perpendicular to the axis, were found to be in the ratio of 

 100 : 79. Considerable qualitative differences were also ob- 

 served in these rays on testing them by coloured glasses. When, 

 however, the plane of polarization was preserved coincident with 

 the optic axis, no matter how the direction of the ray through 

 the crystal might be changed, neither qualitative nor quantitative 



