i88o.] 
Spectrum Analysis. 
741 
in 1815 and 1823, gave lines in strict coincidence with those 
of sunlight, whilst the lines in the spectra of such ot the 
fixed stars as he examined proved to differ m part both from 
those of sunlight and among one another. The light ot 
lamps” showed no dark lines, but, on the contrary, one rig 
yellow line which corresponded in refrangibility to the black 
line marked by Frauenhofer as D in the solar spectrum. . 
Sir D. Brewster, in 1832, observed that, in transmitting 
light through nitrous acid gas, a great number of new black 
lines became apparent in the spectrum a discovery lying 
so close at the root of atomical science,” its author a tew 
years later emphatically remarked, “ that I am persuaded it 
will open up a field of research which will exhaust the 
labours of philosophers for centuries to come.” In 1833 
Prof. Miller, of Cambridge, conjointly with Prof. Darnell, 
noticed a similar behaviour of bromine and iodine vapour 
towards light ; whilst Sir D. Brewster, “ the first and prin- 
cipal object ” of whose inquiries was “the discovery ot a 
general principle of chemical analysis, in which simple and 
compound bodies might be characterised by their action on 
definite parts of the spectrum,” in 1836 added the important 
discovery that the number of dark lines in the solar spectrum 
depended on the thickness of the layer of atmosphere which 
it must traverse to reach the observer, and that, conse- 
quently, some of those lines were owing to a similar ab- 
sorptive action on the part of the. colourless gases of the 
atmosphere as he had previously discovered in red-coloured 
nitrous acid gas. In 1842, moreover, Sir D. Brewster was 
“ surprised to find ” that, “ in the spectrum of deflagrating 
nitre, the red ray, discovered by Mr. Fox Talbot, . . • /occu- 
pied the exact place of the line A m Fraunhofer s 
spectrum,” and he was “ equally surprised to see a luminous 
line corresponding with the line B of Fraunhofer. 1 he 
phenomena of absorption of light, and the consequent pro- 
duction of dark lines in luminous spectra, were elaborately 
investigated by Prof. Miller, of King’s College, m 1845 ; and 
various hypotheses were hazarded, notably one by Baron 
Wrede in 1834, on the mechanical origin of those lines, in 
1840 M. Foucault made an observation which was calculated 
to throw considerable light on the subject, but the bearings 
of which he himself, as well as his contemporaries, excepting 
one, utterly failed to perceive. “ I caused an image of the 
sun,” relates M. Foucault, “ to fall on the [voltaic] arc and 
I convinced myself in this way that the double bright line 
of the arc coincides exactly with the double, dark line [D] of 
the solar spectrum. This process of investigation furnished 
