336 



shape, and that the change of A' in the small region may be 

 neglected. The distribution of tlie light in the scattering line will 

 then also be symmetrical; tliis we may infer without knowing the 

 exact form of the law of darkening '). 



In contrast to what characterizes pure refraction lines, the sym- 

 metry of pure scattering lines is not disturbed by the addition of a 

 similar cause of weakening that is constant in the region considered. 

 (The above expressions (1) and (2) explain this difference). 



Anomalous molecular scattering, or diffusion of light, cannot 

 therefore have any share in the production of the genera! displace- 

 ments of the solai' lines towards the red'). 



Let us now consider the case that our small spectra! region 

 contains tiuo absorption lines. The scattering coefficient will then 

 take the form 



h=C + p 



Nj Nk 



^]' 



32:rt' 



for we may replace the factor by the constant quantity p. 



Fig. 46 represents h as a function of X. To this will correspond a 

 darkening curve whose ordinates grow and decline with h. We see 

 that in the interval between the absorption lines the superposition 

 of their individual scattering effects must produce a greater increase 

 of the darkening than outside tiie pair; so the centres of gravity 

 of the two diffusion-lines will be a little less distant from each other 

 than the absorption lines proper (apparent attraction). 



Summarizing the above qualitative results with a view to their 

 application in the spectroscopy of celestial bodies, we may state: 



1. The general but very unequal displacements of tlie Fraunhofer- 

 lines towards the red can be explained by the properties of refraction 

 lines, but not by those of diffusion lines. This also applies to the 

 limb-centre displacements. 



2. The mutual influence of neighliouring Fraunhofer lines, which 

 increases, as a rule, from the centre towards the limb of the solar 

 disk, may be the result of either scattering process; but irregular 

 refraction causes apparent repulsion, molecular diffusion of light 

 gives apparent attraction. 



1) The law of darkening through molecular scattering in the sun has been amply 

 studied by J. Spijkekboer in a dissertation, published in Utrecht, 1917; cf also 

 Arch, néerl. UIA, 5, p. t-115. 1918. 



') To this point our attention has first been drawn in a conversation with 

 Einstein. 



