Physics. — " The relation between the widening and the mutual 

 influence of disjiersion lines in the spectnun of the sun's limb." 

 By Prof. W. H. Jumüs and Dr. M. Miknaert. 



(Communicated at the meeting of April 28, 1923). 



Introduction. 



*ri)e hypothesis that tlie darkness of Fraunhofer lines is mainly 

 an effect of anonialons dispersion enables one to explain, at any 

 rate qnalitatively, a great many characteristics of the solar spectrnm. 

 It thus appears possible to formulate a theoretical connection — 

 which has then of conrse to be verified quanlilatively — between 

 iinmerous phenomena that are less easily seen as inter-depeiideni if 

 we consider them from the point of view of the nnmodified classical 

 absorption theory introdnced by Kirchhoff. Snch phenomena are 

 e.g.: the general displacement of the solar lines towards tlie red, 

 differing greatly in amount from line to line; the limb-centre dis- 

 placements and their dependence on intensity and wave-lenght ; the 

 widening and the change of intensity of the lines as the limb is 

 approached; the apparent nuitnal repulsion of neighbouring Fraun- 

 hofer lines, generally greater at the limb than in the centre of the 

 disk; the systematically curved shape of the lines of the spot- 

 spectrum if the slit cuts the spot in a direction passing through the 

 centre of the disk; the gradual increase of the distance between 

 the components of the bright calcium lines i/, and K., as the limb 

 is approached ; and var;ous particulars of a more local character. 



We shall endeavour to express mathematically the connection 

 which, according to the dispersion theory, should exist between a 

 few of the above-mentioned phenomena, and then to investigate how 

 far these quantitative relations agree with the results of measure- 

 ments made on solar lines. 



It is evident that the absolute magnitnde of the influence exercised 

 by anomalous dispersion in the solar gases on the aspect of Fraun- 

 hofer lines cannot be calculated directly so long as the refracting 

 and scattering power of the sun is not otherwise known. Neither 

 can this power be safely computed starting from line displacements 

 only. It must be remembered, however, that a similar uncertainty 

 prevails regarding the values given for temperatures, pressures, 

 radial velocities, intensities of magnetic or electric fields, or grades 

 of dissociation in the sun in so far as such values are derived from 



