18 Prof. Kirchhoff on the Relation between the Radiating and 



incident rays whose plane of polarization is parallel to the axis 

 of the plat i', than of those whose plane of polarization is per- 

 pendicular to that axis. If it be granted that the tourmaline 

 retains this property when red-hot, then rays emitted in a direc- 

 tion perpendicular to its surface must be partly polarized in a 

 plane passing through the optic axis, and therefore perpen- 

 dicular to what is called the plane of polarization of the tourma- 

 line. I have experimentally tested this striking deduction from 

 the law here demonstrated, and have confirmed its truth. The 

 tourmaline plates employed bore a considerable heat in a Bunsen's 

 lamp for some time without suffering any permanent alteration, 

 except that they appeared on cooling to have become a little 

 cloudy at the edges. They retained the property of transmitting 

 polarized light even when red-hot, though to a considerably less 

 degree than at a lower temperature. This appeared on observing 

 through a doubly-refracting prism, a red-hot platinum wire 

 placed behind a tourmaline plate. The two images of the wire 

 so produced were of unequal intensity, though the difference 

 between them was much less than when observed through a plate 

 of the ordinary temperature. To the doubly-refracting prism 

 was then given that position in which the difference of the 

 intensities of the two images was a maximum ; if now it was the 

 upper image of the wire that was the brightest, then on removing 

 the wire and observing the plate alone, it was found that the 

 upper image of the plate was unmistakeably though not strikingly 

 duller than the other. The two images appeared exactly like 

 two equal red-hot bodies, of which the upper possessed a lower 

 temperature than the other. 



§ 16. Place must be found for one more deduction from the law 

 here established. If a space be entirely surrounded by bodies 

 of the same temperature, so that no rays can penetrate through 

 them, every pencil in the interior of the space must be so 

 constituted, in regard to its quality and intensity, as if it had 

 proceeded from a perfectly black body of the same temperature, 

 and must therefore be independent of the form and nature of the 

 bodies, being determined by the temperature alone The truth 

 of this is obvious when we reflect that a pencil having the same 

 position but the opposite direction to that chosen, is completely 

 absorbed by the successive reflexions it undergoes from body to 

 body. In the interior therefore of an opake red-hot body of 

 any temperature, the illumination is always the same, whatever 

 be the constitution of the body in other respects. 



It may be observed, by the way, that the proposition demon- 

 strated in this section does not cease to hold good even if some 

 of the bodies are fluorescent. A fluorescent body may be defined 

 as one whose radiating power depends on the rays incident on it 



