SPECTRUM ANALYSIS OF THE HEAVENLY BODIES. 141 
solar spectrum, and was rewarded by the discovery that several of the che- 
mical elements which exist upon the earth, are present in the svlar atmo- 
sphere. 
: The speaker stated that it was his intention on this occasion to bring before 
the Association the results of the extension of this method of analysis by the 
prism to the heavenly bodies other than the sun. These researches have been 
carried on in his Observatory during the last four years. In respect of a 
large part of these investigations, viz. those of the moon, the planets, and 
fixed stars, he has had the great pleasure of working conjointly with the 
very distinguished chemist and philosopher Dr. W. Allen Miller, Treas. R.S. 
The speaker then referred to the principles of spectrum analysis upon 
which their interpretation of the phenomena observed in the spectra of the 
heavenly bodies was based, stating that spectra may be arranged under 
three orders. 
1, The special character which distinguishes spectra of the first order con- 
sists in that the continuity of the coloured band is unbroken either by dark 
or bright lines. We learn from such a spectrum that the light has been 
emitted by an opake body, and almost certainly by matter in the solid or 
liquid state. A spectrum of this order gives to us no knowledge of the che- 
mical nature of the incandescent body from which light comes. 
2. Spectra of the second order are very different. These consist of coloured 
lines of light separated from each other. From such a spectrum we may 
learn much. It informs us that the luminous matter from which the light 
has come is in the state of gas. It is only when a luminous body is free 
from the molecular trammels of solidity and liquidity that it can exhibit its 
own peculiar power of emitting some coloured rays alone. Hence substances 
when im a state of gas, may be distinguished from each other by their spectra. 
Each element, and every compound body that can become luminous in the 
gaseous state without suffering decomposition, is distinguished by a group of 
lines peculiar to itself. It is obvious that if the groups of lines character- 
izing the different terrestrial substances be known, a comparison of these, as 
standard spectra, with the spectrum of light from an unknown source, will 
show whether any of these terrestrial substances exist in the source of the 
light. 
3. The third order consists of the spectra of incandescent solid or liquid 
bodies, in which the continuity of the coloured light is broken by dark lines. 
These dark spaces are not produced by the source of the light. They tell 
of vapours through which the light has passed on its way, and which have 
robbed the light by absorption of certain definite colours or rates of vibra 
tion ; such spectra are formed by the light of the sun and stars. 
Kirchhoff has shown that if the vapours of terrestrial substances come 
between the eye and an incandescent body, they cause groups of dark lines, 
and further, that the group of dark lines produced by each vapour is identi- 
eal in number and in position in the spectrum with the group of bright lines 
of which its light consists when the vapour is luminous. 
It is evident that Kirchhoff by this discovery furnished us with the 
means of interpreting the dark lines of the solar spectrum. For this pur- 
pose it is necessary to compare the bright lines in the spectra of the light of 
terrestrial substances when in the state of gas with the dark lines in the 
solar spectrum. When a group of bright lines coincides with a similar 
group of dark lines, we know that the terrestrial substance producing the 
bright lines is present in the atmosphere of the sun. For it is this substance, 
and this substance alone, which by its own peculiar power of absorption can 
