A QUANTITATIVE DETP:RMINATI0N OF THE ANOMA- 

 LOUS DISPERSION OF SODIUM VAPOR IN THE 

 VISIBLE AND ULTRA-VIOLET REGIONS. 



By U. W. Wood. 



Presented by C. R. Cross. Received May 31, 1904. 



The dispersion of a medium with but a single baud of metallic ab- 

 sorption is represented by the formula 



in which \,„ is the wave-length at the centre of the absorption band, and 

 m a constant. This formula, which is developed from electro-magnetic 

 considerations, is practically identical with the original Sellmeier formula. 

 Up to the present time it has never been tested experimentally, for the 

 reason that no medium of suitable constitution is known which admits of 

 accurate determinations of the variation of the refractive index with the 

 wave-length. We possess such a medium in the vapor of metallic sodium, 

 but the experimental difficulties which stand in our way are so great that 

 only qualitative observations have been made up to the present time. 



The formula given above, being the simplest dispersion formula which 

 we possess, ought by all means to be tested experimentally, and it was 

 with this end in view that the work reported in the present paper was 

 undertaken. 



After a large amount of fruitless work, methods have at length been 

 developed which have made possible accurate determinations of the ab- 

 solute refractive index of the metallic vapor at various densities, for all 

 wave-lengths comprised between the extreme red and the remote ultra- 

 violet. For wave-lengths immediately adjoining the D lines the refrac- 

 tive index has been found to have a value as high as L38, as great as that 

 of some liquids, while the dispersion is so great even at the position of 

 the D3 line of helium, that, could we form a prism of the vapor giving the 

 same deviation as a G0° glass prism, we could by its aid separate a close 

 Zeeman doublet bv an amount as c't^at as the distance between the red 



