Theory to Dark Lines D„ D 2 produced by Sodium- Vapour. 30? 



the density corresponding to the value m=*003. The fol- 

 lowing table gives the breadths of the dark bands for densities 

 of vapour corresponding to values of m from *0002 to fifteen 

 times that value ; and fig. 6 represents graphically the breadths 

 of the dark bands and their positions relatively to the bright 

 lines D 1? D 2 for the first five values of m in the table. 



Values of m. 



Breadths of Bands. 



D r 



D 2 . 



•0002 



•09 



•217 



•293 



•340 



•371 



•392 



•408 



•419 



•11 

 •383 

 •707 

 1-060 

 1-429 

 1-808 

 2-192 

 2-581 



•0006 



•0010 



•0014 



•0018 



•0022 



■0026 



•0030......... 





Fig. 6. 



VmV£S orM 









^n 



J D -° 



002 









j^ 



■ ,( 



006 









5° 2 



_j d ' 









Pd 2 



Jo, 



310 







{j^~ 



1 -0 



014 







_J D ' 











m^ 



018 



§ 7. According to Sellmeier's formula the light transmitted 

 through a layer of sodium- vapour (or any transparent sub- 

 stance to which the formula is applicable) is the same whatever 

 be the thickness of the layer (provided of course that the 

 thickness is at least several wave-lengths, and that the ordinary 

 theory of the transmission of light through thin plates is 

 taken into account when necessary). Thus the D 1? D 2 lines 

 of the spectrum of solar light, which has travelled from the 

 source through a hundred kilometres of sodium-vapour in the 

 sun's atmosphere, must be identical in breadth and penumbras 

 with those seen in a laboratory experiment in the spectrum of 



