343 



a 

 rather great distance, and increases slowly to 0,004 A maximnm. 



Obviously the \'alue of ?*„ has only little intluence on the result. 



As B' differs little from B, the radical quantities of (12) can be 



developed into rapidly converging series. It will then appear that as 



a first approximation the repulsion is proportional to the absolute 



value of the widening at the limb, i.e. to B' — B, Accordingly, our 



curves are also valid for lines differing in width from those here 



considered, provided their widening at the limb has the value found 



by Fabry and Büisson. They are therefore applicable to the case 



of lines having the average type of those for which mutual influence 



has been observed. 



§ 6. Diffuxion-lines in the spectrum of the centre of the solar disk. 



The distribution of the intensity in a pure molecular scattering- 

 line (in the absence of irregular gradients of optical density) depends 

 on the manner in which the scattering coefficient (cf. p. 335): 



C + 



32jr' {rtj—iy 



3X^ Nj 



varies with X in the surrounding small part of the spectrum. And 

 because even the variation of A* may be neglected there, the distri- 

 bution is entirely governed by the nature of 



(nj-\y 



Nj 



= [ ƒ W]% 



a function, obviously symmetrical with respect to the position of 

 the absorption line. On either side of the latter we may again 



^n J)» 



mark a wave-length where (omitting the index ƒ) — — — equals a 



certain — provisionally arbitrary — quantity L*. By these places 

 in the spectrum we define the "jC-boundaries", and by their distance 

 the "L-width" of the diffusion line (Cf. Fig. 6, on p. 338). 



We now introduce the dispersion formula (2) of p. 337 and 

 confine our attention to the case that there is only one single ab- 

 sorption line, so that we may write 



"-'=é^^ ^''^ 



Our two L-boundaries will be found by substituting in this 

 equation n — 1 = ± L l--^ N~ and A =z A/^ or = Xy , which leads to 



^'^-^■^z^ '" ^'-^- = -W • • • ^''^ 



and makes the L-width of the diffusion line equal (o 



