Address of Sir William Thompson. 281 
(2) A very rigorous experimental test of this coincidence by 
Prof. W. H. Miller, which showed it to be accurate to an as- 
tonishing degree of minuteness. 
(8) The fact that the yellow light given out when salt is 
wn on burning spirit consists almost solely of the two 
oat identical qualities which constitute that double bright 
ne 
line D to be absent in a candle-flame when the wick was 
snuffed clean, so as not to project into the luminous envelope, 
and from an alcohol flame when the spirit was burned in a 
watch-glass, And, 
(5) Foucault’s admirable discovery (Z’ Institut, Feb. 7, 1849), 
that the voltaic are between charcoal points is “a medium 
which emits the rays D on its own account, and at the same 
ra absorbs them when they come from another e 
. 
quarter. 
€ conclusions, theoretical and practical, which Stokes 
hich 
el Observations made by Stokes himself, which showed the 
bright flame wh 
lic lectures in the University of Glasgow, were :-— 
(1) That the double line D, whether bright or dark, is due 
to vapor of sodium. 4 
(2) That the ultimate atom of sodium is susceptible of reg- 
ular elastic vibrations, like those of a tuning-fork or of stringed 
musical instruments; that like an instrument with two strings 
d to approximate unison, or an approximately cireular 
elastic disc, it has two fundamental notes or vibrations of ap- 
proximately equal pitch; and that the periods of these vibra- 
Hons are precisely the periods of the two slightly different yel- 
low lights constituting the double bright line D. 
(3) That when vapor of sodium is at a high enough tempera- 
rich light from another source is propagated, its atoms, ac- 
Cording to a well-known general principle of d are 
‘0 vibrate in either or both of those fundamental modes, if some 
5 wept i : d stellar 
(5) That Fraunhofer’s double dark line D of solar and ste 
Spectra is due to the presence of vapor of sodium in atmospheres 
Aw. Jour. Sci.—Tuiep Series, Vo. II, No. 10.—Oct., 1871, 
19 
