350 M. F. Zollner on the Temperature and 



clouds in the penumbra a somewhat darker background becomes 

 visible than in the other parts of the solar surface. Hereby is 

 explained the less brightness of the penumbra in general, as well 

 as the reversal of the relative brightness of the spectral lines 

 which has often been observed in it and at its boundaries. A 

 more precise reason for this phenomenon is obtained from the 

 above-cited experiment of Kirchhoff, and from the causes, fully- 

 discussed in the description of it, by which the relative bright- 

 ness of the spectrum-lines is determined. At the same time, 

 by this view of the constitution and the physical causes of the 

 penumbra, the relatively great variability of its boundaries be- 

 comes explicable. 



§8. 



Let me here be permitted a few remarks on determination of 

 temperature of incandescent bodies in general. According to 

 Kirchhoff' s theorem*, the ratio of the emissive power E to the 

 absorptive power A is a function (the same for all substances) of 

 the temperature t and the wave-length X ; so that, independently 

 of the particular nature of the substance, we have the following 

 relation : — 



B 



A : 



If the substance is not transparent, so that A=l can be put, 

 then is 



E=J„. 



In relation to this yet unknown function, Kirchhoff (/. c, 

 p. 292) remarks ; — " It is a highly important problem to dis- 

 cover this function. There are great difficulties in the way of its 

 determination by experiment ; nevertheless there appear grounds 

 for the expectation that it will be thus ascertained, since it is in- 

 dubitably of simple form, as all functions which do not depend on 

 the properties of individual substances have hitherto been found 

 to be. Only when this problem is solved will it be possible for 

 the whole fruitfulness of the demonstrated theorem to appear." 



In my inaugural dissertation f, which was published shortly 

 before Kirchhoff' s memoir, and an abstract of which appeared in 

 the same volume of PoggendorfPs Annalen%, I have determined 

 photometrically the light emitted by galvanically incandescent 

 platinum wire, and at the same time measured, by observation 

 of the resistance and the intensity of the current, the amount of 



T-J*. 



Ann. vol. oix. p. 291 sqq. 

 f Photometrische Vntersuchungen, insbesondere uber die Lichtentwick- 

 lung gluhender Platindrahte. Basel, 1859. 

 X Vol. cix. pp. 244-275. 



