804 



of the very steep temperature-fall in these small furnaces, the parts 

 of the furnace before and behind the radiating' object were for the 

 greater part considerably cooler than the tnrnialine-plate itself. 



§ 5. After it was demonstrated in this way, that the chosen appa- 

 ratus was really suitable, to make accurate temperature-readings, 

 the other cylindei-, cut parallel to the crvstallographical axis, was 

 tixed into tlie furnace in (piite the same way. By a prelimiuary 

 optical investigation the direction of maximum liglit-absorption was 

 fixed, which direction we will discern as A*,„. The emitted light is 

 elliptically polarised ; the intensity of the radiation foi- vibrations in 

 the two principal directions could be studied easily by rotating the 

 iiicol N. 



a) In the field of the telescope, R„i 'iiay be in a vertical position. 

 The polarisator had its plane of vibration parallel to /^«; reading at 

 739° C. : 350 M.A. If the polarisator .V is rotated over 90°, the intensity 

 of the current in the incandescent-lamp is only 344 M.A. at 739° C. 

 When the nicol is rotated over 360°, the following readings were 

 made in the four principal situations: 350 M A.; 344 M.A.; 351M.A.; 

 346 M. A. ; finally once more. 351 M.A. 



The apparent temperature of the small cylinder with respect to 

 vibrations in the direction of maximum absorption thus seems to be 

 clearly higher for 14° or 15° C, than in a direction perpendicular 

 to the first. 



b) Now R„, was in a horizontal position; the plane of vibration of 

 the prism ^V is vertical. At 751° C. the readings were now: 351M. A., 

 and after A^ being rotated over 90^, — 356 M. A. 



c) R,n is replaced as in a); the polarisator has its plane of vibration 

 parallel with R,n. Readings : at 756' C, first 358 M.A., and after 

 rotating y over 90°: 352 M.A. 



cl) R,„ is again horizontal. At 815° C. the readings of the milli- 

 ampèremeter are : 376 M.A. and 381 M.A., according to the plane 

 of vibration of N being perpendicular to, or parallel with R,n. 



e) R,n is now in a vertical position. Readings at 826° C. : 388 M.A. 

 and 383 M.A. If the nicol is removed, then the reading is in all 

 directions ; 402 M.A. ; the apparent increase of temperature is of course 

 explained by the light-absorption of nicol-, and prism-system. There- 

 fore all numbers of M.A., as they are found, need to be augmented 

 with 20 M.A., to get the true radiation-temperatures (Table). 



/) The experiments mentioned d and e were now repeated, with 



