134 INFRA-RED EMISSION SPECTRA. 



constant 5=2.5 gram calories per minute = 0.175 Xio 7 ergs per square 

 centimeter per second, R = 0.805X10" ergs. 



If we equate the sun's radiation to od i , where a is the radiation constant, 

 we get 6, the "effective temperature" of the sun, that is, the temperature 

 of a full radiator which is emitting energy at the same rate. 



Thus, 5.35X10 -5 6 i = 0.805 Xio 11 ; whence 6=6200 abs. 



With each new determination, however, the solar constant is being 

 reduced from the former high value, so that a more probable value 1 is 

 about 2.1 gram calories per square centimeter per minute. 



Using this value of 5= 2.1 = 0.147 Xio 7 ergs per square centimeter per 



second, 



7t = o.o676Xio u , 



whence 



0=5980 abs. 



This is somewhat closer to Wilson's 2 value (5773 abs.), which he 

 obtained by making a direct comparison of the radiation from the sun 

 with that from a "full radiator" 3 at a known temperature. 



Warburg 4 has also computed the probable temperature of the sun. 

 He assumes that the rate of radiation per degree is constant, and that the 

 Stefan-Boltzmann law is applicable at all temperatures. For 5=2.54 

 gram calories per second, the computed temperature is 6256 abs., and for 

 5=2.17 gram calories per second, a temperature of 6014 abs. 



Fery and Millochau 5 have measured the temperature of the solar surface 

 with a pyrometer. The mean value of the observed temperature, after 

 correcting for atmospheric absorption, is 5620 abs. For different parts 

 of the solar disk the temperatures vary from 5888 to 5963 abs. 



Abbot (loc.cit.) gives a solar spectrum energy curve, deduced from 

 bolometric measurements, in which the maximum occurs at 0.49 ti, with 

 a possibility of the maximum 8 lying at about ^ = 0.46/*. The equation 

 ^ max T= const. = 2920, using the value of ^=0.46//, gives a black-body 



1 Abbot : Smithsonian Miscellaneous Collections, vol. 45, p. 81, 1903. See also his later 

 results, Annals Astrophys. Obs., vol. 2, p. 99. 



2 Wilson: Proc. Roy. Soc, vol. 69, p. 312, 1901. 



3 Poynting (loc. cit.) very aptly says: "A surface which absorbs and therefore emits every 

 kind of radiation, is usually described as 'black', a description which is obviously bad when 

 the surface is luminous. It is much better described as 'a full absorber' or 'a full radiator,'" 

 i.e., a complete radiator, as distinguished from a partial radiator or so-called "non-black 

 body." In justice to Kirchhoff who was the first to give a clear discussion of this subject, it 

 should be called the Kirchhoff radiator. 



4 Warburg: Verb. Deutsch. Phys. Ges., I, p. 50, 1899. 



6 Fery & Millochau: C. R., 143, pp. 505, 570, 731, 1906. 



6 The curve as drawn by Abbot is very asymmetrical and depressed on the side of the short 

 wave-length. From the data given, it is possible to draw the radiation curve more symmetri- 

 cal, as one would expect it to be, which shifts the maximum to about 0.46 n. See also his later 

 results, Annals Astrophys. Obs., vol. 2, which have been published since writing this paper. 



