335 



distant from each other than their cores, i.e. the true absorption 

 lines: we observe an apparent repulsion. 



In Fig. 3 we see, moreover, that on the violet side of each line 

 there appears a point where n — 1=0. (If «„ — 1 were negative, 

 such a point wonid be found on the red side of the line). Light of 

 the corresponding wave-length would not be weakened by irregular 

 refraction and should, therefore, show an intensity in the spectrum, 

 surpassing the average intensity of regions clear from lines. Jewell') 

 seems indeed to have observed casually such phenomena in the 

 solar spectrum. It is not sur[)rising, however, that similar places of 

 greater brillancy are not very conspicuous there ; for we can scarcely 

 doubt that in the sun the proportion of (he components of the 

 mixture varies with depth, so that the values of 1 for which 

 n — 1 = will not be the same on the entire paths of the beams. 

 Moreover, the Fraunhofer lines are partly due to molecular scat- 

 tering, and it will presently he shown that this process does not 

 involve the appearance of such narrow regions of greater brillancy 

 in the spectrum (at least not in the central parts of the solar disk). 

 Both circumstances tend to obliterate the brighter places near refrac- 

 tion lines. 



B. Imaginary pure scattering-lines. 



Now suppose the density of a gaseous mixture to be so uniform, 

 that rays of light pass through it in straight lines, then the true 

 absorption lines will rjet be enveloped into dispei'sion lijies, because 

 for kinds of light belonging to the nearest environment of the 

 distinctive frequenties the coefficient of molecular scattering has 

 greater values. Let us analyse, indeed, how 



3 X* Ni 



varies with J in a narrow spectral region containing a single absorp- 

 tion line of the constituent ƒ. All terras of the sum but one may 

 there be treated as constants, so that 



This quantity vai'ies with 1 in the manner represented in Fig. 4, a ; 

 the curve is symmetrical with respect to the absorption line, provided 

 that the dispersion curve associated with the line has the regular 



') Jewell, Astroph. Journ. Ill, 99, 1896. Cf. also: Abbot, The Sun, p. 115, 

 where analogous observations of Eveeshed are mentioned in addition. 



