12 ON THK Al'l'LJCATlON OF INTERFERENCE METHODS 



It follows that the \'isibihty curve is practically the same as that due to 

 a double source, whose components have the intensity ratio 7:10, and in each 

 of which the Ught is distributed according to the exponential law expressed 



l)y the fii-st term. 



The fonnula for a doul)le source, where the components are similar, is 



l+r^+2rcos2r.^ 



* l+r"+2r 



in which JJ, tlie period of the cm've, is inversely proportional to the distance 

 between the components. 



But D = .V/.i = (-V+l) >-2, whence a = Aj - l^ = ^ • 



Hence, in the present instance we have for the distance between the com- 

 ponents of the red hydi-ogen Hne, 1 30 x (6.56x10-*)^ = 1.4 x lO-«""», or 0.14 

 divisions of Rowland's scale. 



Again, if 'j be the " half- width " of the spectral Une (the value of x when 



(f{x) = i), then 



'^(j-) = 2~52, and V=e r2~' 



If A be the value of X for F= i, then 8 = - , or, with sufficient accu- 



TT A 



. 0.22 

 racy, ^^ = ^-- 



Substituting the value of cl in the equation for T^ we have 



V=2-Er 



The value of a in the hydrogen curve is 19. Accordingly, after reducing 

 to the same units as above, we have 5 = 0.049. 



From these data. Fig. 3rt was constructed, the lull cm-ve sliowing the dis- 

 tril)utiou of hght in the source. 



Fig. 4/>, Plate II., gives, in the full cui-ve, the corrected values of the visi- 

 bility of the blue hydrogen line at the same temperatui-e and pressm*e as 



• Ah frequont use is to be uiade of the function 



A /l+r»+2r«)«2i:^ 

 it will be abbreviated to the fonn ews r, 7A 



