Theory to Dark Lines D l5 D 2 produced by Sodium- Vapour. 305 



malous dispersion by prisms of sodium-vapour *, by which 1 

 was astonished and delighted to see not merely a beautiful and 

 perfect demonstration of the " anomalous dispersion " towards 

 infinity on each side of the zero of refractivity, but also an 

 illustration of the characteristic nullity of absorption and 

 finite breadth of dark lines, originally shown in Sellmeier's 

 formula t of 1872 and now, after 27 years, first actually 

 seen. Each photograph showed dark spaces on the high sides 

 of the D 1? D 2 lines, very narrow on one of the photographs; 

 on the other much broader, and the one beside the D 2 nne 

 decidedly broader than the one beside the D l line ; just as it 

 should be according to Sellmeier's formula, according to 

 which also the density of the vapour in the prism must have 

 been greater in the latter case than in the former. Guessing 

 from the ratio of the breadths of the dark bands to the space 

 between their D„ D 2 borders, and from a slightly greater 

 breadth of the one beside D 2 , I judged that m must in this 

 case have been not very different from *0002 ; and I calculated 

 accordingly from (6) the accompanying graphical represen- 

 tation showing the value of 1 , represented by y in fig. 1. 



Fi g'- !• w=-0002. 























s 



t 









































~\w 





D, 







































1 











































I 











































1 











































1 I 

























\L 



. 



9< 



6 



9< 



)7 



9< 



>8 



9< 



>9 















A 



ymp 



tote 



?=Q 



ooz 



_y 





y 



0,A 



iym, t 



vtott 











> 



I0( 



t\ 



f 



001 



1 



002 



i 



003 









A 























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3 \ 



n 1 



























































































































































































































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* A description of Professor Becquerel's experiments and results will 

 be found in Comptes Rendus, Dec. 5, 1898, and Jan. 16, 1899. 



t Sellmeier, Pogg. Ann. vol. cxlv. (1872) pp. 399, 520 ; vol. cxlvii. 

 (1872) pp. 387, 525. 



