524 Proceedings of Royal Society of Edinburgh. [sess* 
turbed ether • v s the velocity of light of period r in the sodium- 
vapour. 
§ 3. For sodium-vapour, according to the measurements of 
Rowland and Bell,* published in 1887 and 1888 (probably the 
most accurate hitherto made), the periods of light corresponding to 
the exceedingly fine dark lines D 1} D 2 of the solar spectrum are 
*589618 and *589022 of a michron.t The mean of these is so 
nearly one thousand times their difference that we may take 
«=*(«+ *0( i - 26 oo) ; K ‘-i( K + K l( 1 + mo) ■ (2) ■ 
Hence if we put 
= i(K + K ' ) ( 1+ lM0, 
X \ 
7 
• • • ■ ( 3 ); 
and if x be any numeric not exceeding 4 or 5 or 10, we have 
whence 
1 
1000 
(2a; +1); 
Bi 
1000 
(2x - 1) . (4); 
r 2 1000 . r 2 1000 
r 2 - k 2 ~2x +1’ r 2 - K y 2x - 1 
(5). 
Using this in (1), and denoting by g the refractive index from 
ether to an ideal sodium- vapour with only the two disturbing atoms 
m, m /5 we find 
1000m 
2a; + 1 + 
1000m / 
2x —1 
( 6 ). 
§ 4. When the period, and the corresponding value of x accord- 
ing to (3), is such as to make g 2 negative, the light cannot enter 
the sodium-vapour. When the period is such as to make g 2 
positive, the proportion, according to Fresnel and according to the 
most probable dynamics, of normally incident light which enters 
the vapour is 
* Rowland, Phil. Mag., 1887, first half-year ; Bell, Phil. Mag., 1888, first 
half-year. 
t ‘ ‘ Michron ” is the name which I have given to a special unit of time 
such that the velocity of light is one mikrom of space per michron of time, the 
mikrom being one millionth of a metre. The best determinations of the velo- 
city of light in undisturbed ether give 300,000 kilometres, or 3 x 10 14 mik- 
roms, per second. This makes the michron i x 10 - 14 of a second. 
