( <^^- ) 



the photospliere would show itself as an equally luminous disk. Hut 

 then it appears to be impossible to find such values for the thick- 

 ness of that atmosphere and for its coefficient of absorption, as to 

 give a law for the rate of diminution of brightness, consistent with 

 observation. Very ') e.g. when attributing the effect to absorption 

 only, arrives at the absurd result that we should have to assume 

 that the absorptive power toward the limb is smaller than tliat nearer 

 the center. He, therefore, suggests the existence of other influences 

 which, combining with the absorbent process, would reconcile theory 

 to observed facts. Diffraction by fine particles, columnar structure 

 of the solar atmosphere, irregularity of the photospheric surface, are 

 thus introduced. 



ScHCSTER '), on the other hand, is of opinion that the difHiculty 

 which bas been felt in explaining the law of variation of intensity 

 across the solar disk is easily removed by placing the absorbing 

 layer sufBciently near the photosphere and taking account of the 

 radiation which this layer, owing to its high temperature, must itself 

 emit. He tiien really finds \alues for the absorption and the emission 

 of that layer, harmonizing with the results of Vf.ry's and Wilson's ') 

 measurements, and also with the properties of the energy curve of 

 the spectrum of a black body at different temf)eratures. But, for all 

 that, serious doubts as to the correctness of the premise and the 

 conclusions must subsist. 



' Indeed, the calculations of Schuster as well as those of Very, 

 Wilson, Langley, Pickering and others, concerning the same subject, 

 are based on the assumption that the light travels along straight 

 lines through the solar gases, whereas everybody who has duly 

 noticed A. Schmidt's "Strahlenbrechung auf der Sonne" will at the 

 least have to give in that rays coming from the outer zones of the 

 disk must have followed curved paths through the solar atmosphere. 

 By this circumstance the said calculations lose their convincing power. 



And besides, the fundamental idea that a considerable portion of 

 the photospheric radiation should be absorbed by a thin atmosphere, 

 encounters a difTiculty of greater importance still This point, I 

 think, has also first been moved by A. Schmidt. What becomes of 

 the absorbed energy accumulating in the atmosphere ? According to 

 Schuster e.g. (I.e. p. 322) the atmosphere transmits largely Vs of 



1) F. W. Very. The absorptive power of the solar atmosphere. Astroph. Journ, 

 16, p. 73-91, (1902). 



«) A. Schuster. Astroph. Journ. 16, p.320— 327, (1902); 21, p. 258— 261, (1905). 



3) W. E. Wilson and A. A. Rambaut. Proc. Roy. Irish Acad. [3], 2, p. 299— 

 334, (1892). 



