330 
nection with another calculation of which more will be said in 
subsequent sections but for our present purpose it will be seen to 
be immaterial. 
Let us try to represent the dispersion of sodium-vapour by 
means of a ^-electronic formula, no account being taken of ex¬ 
tinction. According to the well-known measurements due to Row¬ 
land and Bell we assume 
I A 01 = 5,89618 . IO“ 5 cm. 
I X 02 = 5,89022.10-5 cm. 
Let 
(2) oq = e ± 2 N ± / 3 u c 2 a 2 = e 2 2 X 2 / 3 u c 2 m 2 . 
We have then, by § 6., 
(3) B :==z a i L 1 -j- (%21*2 • 
Here B ; L ly L 2 have the following meaning: 1) for values of À<^À 02 
T _ V* . r _ 
(4) B — Ll — ~ r ~ i r * ’ L 
2) for values of / > x 01 
/pL\ p_ 1,2 1 . J _ ^01 2 ^ 2 . j _ ^02 2 
(5) B - . , Ly - y, î . 1 ^2 - 
v 2 -\-2 
1* - V 
If we put 
(6) a ,-a, = ß-, T ^ T . = X 
A+ 4 
equation (3) may be written 
r=a 1 + /?X. 
Li+L 2 
In the following table the wave-lengths (in 10 -5 cm.) are given in 
the first column, the observed indices in the second column, in the 
third and in the fourth the corresponding values of X and Y. Wave¬ 
lengths smaller than 5,75 have not been included for two reasons. 
In the first place, we should probably not be justified in assuming 
that the pair of the ZMines controls exclusively the dispersion of 
sodium-vapour beyond a certain distance in the spectrum; besides, 
as has been explained by Professor Wood himself, the observed 
