1898-99.] 
Lord Kelvin on Sellmeier's Theory. 
525 
1 - 
>-l 
,/x + 1 
)' 
( 7 ), 
if the transition from space, where the propagational velocity is v ei 
to medium in which it is v s , were infinitely sudden. 
§ 5. Judging from the approximate equality in intensity of the 
bright lines D x , D 2 of incandescent sodium- vapour ; and from 
the approximately equal strengths of the very fine dark lines 
D 1? D 2 of the solar spectrum ; and from the approximately 
equal strengths, or equal breadths, of the dark lines D 1? D 2 
observed in the analysis of the light of an incandescent metal, or of 
the electric arc, seen through sodium-vapour of insufficient density 
to give much broadening of either line; we see that m and m / 
cannot be very different, and we have as yet no experimental 
knowledge to show that either is greater than the other. I have 
therefore assumed them equal in the calculations and numerical 
illustrations described below. 
§ 6. At the beginning of the present year I had the great 
pleasure to receive from Professor Henri Becquerel, enclosed with 
a letter of date Dec. 31, 1898, two photographs of anomalous 
dispersion by prisms of sodium-vapour,* by which I was astonished 
and delighted to see not merely a beautiful and perfect demonstra- 
tion of the “ anomalous dispersion ” towards infinity on each side 
of the zero of refractivity, but also an illustration of the character- 
istic nullity of absorption and finite breadth of dark lines, originally 
shown in Sellmeier’s formula f 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 line decidedly broader than the one beside the D x 
line ; just as it should be according to Sellmeier’s formula, accord- 
ing 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 15 D 2 borders, and from a slightly greater 
* A description of Professor Becquerel’s experiments and results will be 
found in Comptes Eendus, Dec. 5, 1898, and Jan. 16, 1899. 
t Sellmeier, Pogg. Ann., vol. cxlv. (1872) pp. 399, 520 ; vol. cxlvii. (1872) 
pp. 387, 525. 
