OF RADIANT HEAT THROUGH CRYSTALS. 103 



A comparison of these numbers leads us to the conclusion, 

 tJiat the differences between the quantities transmitted are greatest 

 when the heat is polarized in the manner last described, less in 

 the case of the natural rays, and vanish altogether when the rays 

 are polarized in the manner first described. 



The essential difference between the two cases w^here polarized 

 heat was applied is to be traced to the fact, that in the first case 

 the plane of polarization coincides with the axis of the crystal, 

 whereas in the second case the axis was perpendicular to the 

 plane of polarization. The question now arises, whether dif- 

 ferences exist in the quantities transmitted along the axis, when 

 the heat is polarized in different planes. To solve this point the 

 crystal was placed with its axis in the line of transmission, the 

 principal section of the Nichol was set successively vertical and 

 horizontal, and the quantities of heat transmitted in both cases, 

 respectively, were measured and compared. No difference which 

 w^ould permit of the conclusion that the quantity transmitted 

 along the axis was at all affected by the position of the plane of 

 polarization, was observed*. 



When the transmissive power in different directions perpendi- 

 cular to the axis of the crystal was determined, no difference was 

 perceptible, it mattered not whether the light was in its natural 

 or in a polarized condition. For all these directions the same 

 result was obtained as that already communicated for one of 

 them. 



The foregoing results rendered another question of interest to 

 me, and that is, whether the calorific rays whose quantitative dif- 

 ferences were exhibited in the foregoing experiments, have also 

 qualitative differences impressed upon them by passing,in the one 

 or the other direction, through the crystals above-mentioned f. 



* Slight differences in the thermoscopic results were sometimes observed 

 according as the principal section of the Nichol was vertical or horizontal ; but 

 this was solely due to the slight disturbing influence caused by the elliptic 

 polarization of the rays already noticed. Even without the rock-crystal, beryl, 

 or tourmaline, these differences showed themselves when the Nichol was turned ; 

 and the ratios of the observed values were not in the least altered by the intro- 

 duction of the crystal in the manner indicated, — a result established in the most 

 decided manner by exact measurements in the case of each. 



t In my earlier experiments with transparent calcareous spar no such dif- 

 ference was observable. — Pogg. Ann. vol. Ixxiv. pp. 185, 18G. 



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