abbot] studies of solar constant of radiation 83 



fixing the wave-length of maximum energy at about 0.49//.^ Their 

 agreement would be more exact, there can be little doubt, if it were 

 not for the large and variable absorption of the silvered surfaces in 

 the optical apparatus for wave-lengths at and beyond the region of 

 maximum energy. The transmission of the spectroscope at a wave- 

 length of 0.45/^ has varied on this account at dififerent times from 

 33 percent to 15 percent, whereas at wave-lengths of i//. and there- 

 abouts the transmission always approaches 90 percent. The spectro- 

 scope mirrors arje resilvered about once in two months and the 

 siderostat mirrors still oftener. 



Paschen has empirically derived a law connecting temperature 

 with wave-length of maximum radiation, which is expressed as fol- 

 lows, where T is the absolute temperature and ?.^^^^_^, the wave-length 

 of maximum intensity of radiation expressed in microns : 



nuu 



T= constant. 



The value of this constant for the radiation of a " black body " or 

 perfect radiator as determined by Paschen,^ Lummer and Prings- 

 heim,^ and others is about 2900, while for bright platinum Lummer 

 and Pringsheim give 2630 with values for other substances inter- 

 mediate between these. 



Taking the higher value in connection with the observed position 

 of maximum in the solar energy curve outside the atmosphere, we 

 find that the sun's radiation may be assumed comparable as regards 

 the wave-length of maximum radiation to the emission of a " black 

 body " at 5920° absolute. Readers will draw their own conclusions 

 as to the probability that the solar temperature actually lies near 

 this value, but it may be remarked that a further correction of the 

 energy spectrum curve for the selective absorption of the solar 

 envelope would undoubtedly reduce the wave-length of maximum 

 radiation still further, and would thus incline us to the view that 

 the interior of the sun is at a higher temperature than the above con- 

 siderations alone would indicate. 



* The wave-length of maximum energy determined by Mr. Langley on 

 Mount Whitney was about 0.52,". 



^ Verhandlungcn d. Deutschcn Phys. Gcs., iii, 2>7, IQOI- 

 ' Paschen, Astrophysical Journal, ix, 306, 1899. 



