INDEXED 
THE 
LONDON, EDINBURGH, ann DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENCE 
[SIXTH SERIES.] 
SEPTEMBER 1904. Yr ~% 
one &, 
Sa 
SX A, “a 
: . . . BS Se CE 
XXXII. A Quantitative Deternunation of the Heaton ek is: 
persion of Sodium Vapour in the Visible and Ultra-violet ~~ 
Regions. By R. W. Woon, Professor of Experimental 
Physics, Johns Hopkins University ™. 
[Plates X.—XIT. |] 
ee dispersion of a medium with but a single band of 
metallic absorption is represented by the formula 
m=1l+ a ee 
M—)?? 
in which d,, is the wave-length at the centre of the absorption- 
band, and maconstant. ‘This formula, which is developed 
from electromagnetic 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 vapour 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 
* Communicated by the Author. 
Phil. Mag. 8. 6. Vol. 8. No. 45. Sept. 1904. Xx 
