800 



meters \vill indicate only that temperature, whieli an tibsoliitely black 

 body should possess, to show the same emission, that really is 

 observed by means of the pyrometer. Just because different objects 

 difïer from the absolutely black state in an unequal degree, they will 

 seem to possess different radiation-temperatures, when heated to the 

 same temperature of f C. 



If the radiating object, as in the case of birefringent crystals, is 

 anisotropous with respect to its absorption for radiant energy, it must 

 be also anisotropous with respect to its emission, in accordance with 

 Kirchhoff's fundamental law. Such a radiant anisotropous body will 

 behave therefore as if it had different temperatures in different 

 directions of vibration; its apparent radiation-temperature will not 

 be the same for different vibration-directions of its emitted radiation. 



§ 2. Although this conclusion from Kirchhoff's law of radiation, 

 has been tested already occasionally by means of experiment ^), 

 — as we learned however just after this investigation was finished, — 

 all those experiments were made at a time, when the construction 

 of radiationpyrometers, founded on the law of Kirchhoff, and on 

 those of WiEN and Planck formulated since that date, had not yet 

 taken place : We thought it interesting, to demonstrate the said 

 phenomenon once more by means of a radiationpyrometer, as it is 

 used now in a very perfectly de vel loped form in all laboratories for 

 high temperature work, and thus to show at the same time again 

 the validity of Kirchhoff's law, in qualitative respects, by means of 

 a striking experiment. 



§ 3. Our experiments were made in the following way. 



From a crystal of dark green turmaline of Brasil, two small flat 

 cylinders of about I mm. thickness were prepared ; one of them had its 

 axis parallel to the crystallographical axis of the trigonal mineral, 

 the other one perpendicular to it. The form of a cylinder was chosen. 



1) Kirchhoff himself (Pogg. Ann. 109, 299. (I860)), has already drawn this 

 conclusion from liis tiieory, and tried to demonstrate it by experimenting with a 

 heated turmalinecrystal. The same experiment was repeated later on by Balfour 

 Stewart (Phil. Mag. (4). 2, 391. (1861)). Although both experiments can be 

 considered as proving the fact, they are not adapted to make a strong impression. 

 In 1902 however the law of Kirchhoff for this case was demonstrated in a 

 convincing and quantitative manner by Pflüger (Ann. d. Physik (4). 7, 106 (19U2) ), 

 who measured by means oi a spectrophotometer, as well the difference in absorption 

 for the ordinary and extraordinary lightwaves, as the difference in emission of 

 white-hot turmaline tor vibrations in the direction of the crystallographical axis, 

 and for those perpendicular to it. 



