388 Sir David Brewster on the Lines of the Solar Spectrum, 
the solar spectrum, so as to enable us to measure their refrac- 
tive and dispersive powers with minute accuracy, whereas the 
gaseous lines can be rendered visible, however imperfectly the 
spectrum may be formed. In determining the various ele- 
ments of double refraction and polarization, and in all optical 
researches where the phenomena vary with the refrangibility 
of the rays, the gaseous lines will hereafter perform a most 
important part. 
Had the solar lines been much broader than they are, we 
might have been able, by means of minute thermometers, to 
have ascertained the temperature of all those parts of the 
spectrum where there was no light, and thus to have deter- 
mined whether or not the rays s of light and heat are separate 
and independent emanations. The ‘phenomena of the nitrous 
acid gas spectrum, the lines of which can be widened at plea- 
sure, “enable us to perform this and other interesting experi- 
ments, and thus to decide many important questions in the 
theory of radiant matter. 
From the various experiments which I had made on the ae 
sorptive action of coloured media, I was led to a general prin- 
ciple, which, in that stage of the inquiry, appeared to possess 
considerable importance. The points of maximum absorption 
exhibited a distinct coincidence with some of the principal 
dark lines in the solar spectrum, and thus indicated that these 
lines marked, as it were, weak points of the spectrum, on 
which the elements of material bodies, whether they existed 
in the solar atmosphere or in coloured solids and fluids, ex- 
ercised a particular influence. ‘These actions, however, were 
so indefinite, that, with the exception of the oxalate of chro- 
mium and potash*, a salt of most remarkable properties, they 
never appeared in the form of lines or distinct bands. ‘The 
light which was left shaded into the dark spaces, and there- 
fore) notwithstanding the general coincidence which I had 
observed, the phenomena of ordinary absorption could not be 
identified with those of the definite actions by which the solar 
lines are produced. i 
This point of similarity, however, led me to institute a di- 
ligent comparison between the solar lines and those of the 
nitrous acid gas spectrum; and it did not require many ex- 
periments to prove, that there existed between these two 
classes of phanomena a most remarkable coincidence. — In 
order to afford ocular demonstration of this fact, I formed 
the solar and the gaseous spectrum with light passing through 
the same aperture, so that the lines in the one stood opposite 
* [See Lond. and Edinb. Phil. Mag.,, vol. ii. p. 362 ; and vol. vii. p. 436.] 
