33Ó 



spectral phenomena; in tact, sncli statements are alwavs based on 

 the donbtful assumption that the observed spectral phenomena are 

 entirely due to the causes mentioned. There is, of course, no 

 objection to introducing this assumption, — provided its hypothetical 

 character be always kept in mind. 



With equal justification we may assume that Fraunholer lines 

 are mainly "dispersion lines"; the essential point will then be to 

 examine whether the deductions from this hypothesis result in an 

 adequate theory, covering a substantial proportion of observational 

 data. In this paper we confine ourselves to showing that the dis- 

 persion theory of the solar spectrum connects quantitatively two at 

 first sight independent groups of observed |)heuomeua, namely the 

 well-established general tvidening of the Fraunhofer lines at the 

 limb, and the increase, also at the limb, of the vmtual influence of 

 neighbouring lines. This relation proves to be independent of the 

 unknown laws that govern the weakening of any given kind of 

 light on its way through the solar gases; it enables ns to indicate 

 an tipper limit of the mutual influence that may be expected, thus 

 lending fresh support to our fundamental hypothesis. It will lie 

 shown, indeed, tiiat the average value of the mutual influence as 

 deduced from the dispersion theory is perfectly consistent with the 

 actual observations. 



The dispersion lines which, according to our hypothesis, envelop 

 the exceedingly narrow') true absorption lines of the solar spectrum 

 arise from two dimming processes, viz.: irregular lefVaction and 

 molecular scattering. For although light of any wave-length is sub- 

 ject to refraction and scattering on its long way through selectively 

 absorbing gases, it is well known that these causes of darkening 

 specially affect waves in the immediate vicinity of absorption lines. As 

 the two [irocesses weaken the transmitted light according to different 

 laws, we shall treat them separately. 



I. On the weaken'ing of light in passing through 



EXTENSIVE MASSES OF GAS. 



^ 1. Spreading of light by irregular ray-curving in a mixture 

 of gases. 



Suppose we have in a given space a mixture of gases which, if 

 they were each of them alone to fill the space, would show the 

 absolute refractive indices n^, n, . . . . Ui . . . .; then, according to ex- 



') Our assumption that real absorption is restricted to very small ranges of 

 wave-lengths is in harmony with views recently derived from tlie quantum theory 

 by N. Bohr (Zeitschr. f. Pliysik 13, 162, 1923). 



