3:880.] Spectrum Analysis . 743 
explanation of the origin of Fraunhofer’s lines, and au- 
thorise conclusions therefrom respecting the material consti- 
tution of the atmosphere of the sun. ... I formed a solar 
spedtrum by projection, and allowed the solar rays concerned 
... to pass through a powerful salt-flame. If the sunlight 
were sufficiently reduced there appeared in the place of the 
two dark lines D two bright lines ; if, on the other hand, its 
intensity overpassed a certain limit, the two dark lines D 
showed themselves in much greater distinctness than with- 
out the employment of the salt-flame. The speCtrum of 
the Drummond light,” originally continuous when trans- 
mitted through a salt-flame, exhibited “ two dark lines of 
remarkable sharpness and fineness, agreeing with the lines 
D of the solar speCtrum.” All this was similar to the pre- 
vious observations of M. Foucault ; but, whilst the latter 
drew no inferences from his faCts, M. Kirchhoff immediately 
concluded from his observations “ that coloured flames, in 
the speCtra of which bright sharp lines present themselves, 
so weaken rays of the colour of these lines, when such rays 
pass through the flames, that in place of the bright lines 
dark ones appear as soon as there is brought behind the 
flames a source of light of sufficient intensity.” And he 
adds with emphasis “ I conclude, further, that the dark 
lines of the solar speCtrum which are not evoked by the 
atmosphere of the earth, exist in consequence of the pre- 
sence, in the incandescent atmosphere of the sun, of those 
substances which, in the speCtrum of a flame, produce 
bright lines at the same place. . . . The dark line D in the 
solar speCtrum allows us, therefore, to conclude that there 
exists sodium in the sun’s atmosphere. Brewster has found 
bright lines in the speCtrum of the flame of saltpetre at the 
place of Fraunhofer’s lines A, a, B ; these lines point to the 
existence of potassium in the sun s atmospheie. 
Already, from the researches of Leslie, of the year 1804, 
and still better from some observations by Mr. Balfour 
Stewart published in 1858-9, the proportionality between the 
absorptive and emissive powers of substances for rays had 
resulted,— at least so far as radiant heat or invisible rays 
are concerned ; and, whilst a mechanical explanation, ap- 
plicable in general, of this law, as we have seen, had been 
deduced by M. Angstrom, from a hypothesis of Euler’s, Mr. 
Stewart, in i860, had roughly proved the same to hold good 
also in the case of visible rays or light. M. Kirchhoff has 
attempted to furnish a stria mathematical demonstration of 
the necessity of its validity ; and whatever opinion may be 
held of the value of this proof, the law itself is sufficiently 
