Fan. 12, 1882] 
NATURE ; 
253 
ination of the sun’s limb, we may be able to form an idea as to 
whether such hydro-carbons are present in the atmosphere of the 
sun. Of course, as I said before, this work is new and will re- 
quire a great deal of confirmation before it can be put forwards 
an absolute certainty. Mr. Lockyer suggested another way in 
which this broadening of the lines of the limb might arise. Sup- 
posing the corona were to be composed of hydro-carbon, and 
also supposing; we got a streamlet or streamer of the corona 
lying in the same line to the earth as the limb of the sun; then 
we should get exactly the same result. Studying the diagrams, 
it is quite evident that if it were composed of hydro-carbon we 
might get a thickening of the hydro-carbon lines at the limb, 
because it would pass through a greater quantity of matter than 
at the centre. 
Secondly, these hydro-carbons might exist in our atmosphere ; 
but then if they did exist there, there is a very easy way of 
Fic. 16.—Solar corona, as seen during an eclipse. 
proving whether such is the case. 
spectrum when the sun is very high in the heavens, and photo- 
graph it also when the sun is very low, we pass through a very 
much larger quantity of atmosphere in the second case than we 
do in the first case. 
been any particular absorption with a low sun, except, it may 
be perhaps, a greater general absorption on the more refrangible 
side of the dark Zline. But this, of course, like the rest of 
the work, requires still further examination, and I am only, as 
it were, throwing out hints as to the work that will have to be 
done in this particular region. 
But now about aqueous vapour. In 1860 Brewster and Glad- 
Now I have never noticed that there has | 
stone published a map of the spectrum, showing atmospheric | 
absorption, and this investigation was continued by an eminent 
French physicist, M. Janssen, in 1864. Brewster and Gladstone 
had shown that certain lines were due to atmospheric absorption ; 
and Janssen went farther and tried to confirm their researches, or 
‘stanky li 
If we photograph the solar | to correct them, 
He lit a bonfire near Geneva, and observed 
the spectrum of the bonfire thirteen miles off, and found that 
there were a great many lines in the spectrum when he observed 
| it thirteen miles off, which were not present when he ob- 
served it near; and these could only be due to atmospheric 
absorption, But that did not settle the point whether the lines 
that he noticed were due to the air or to aqueous vapour, In 
order to ascertain which lines were due to aqueous vapour, in 
1860, he filled an iron cylinder 330 feet long with steam, and 
then observed the absorption spectrum of steam through this 
great length of tube, and he noted that certain lines were visible 
in the spectrum which were also visible in the spectrum which 
he obtained by viewing the bonfire thirteen miles off. As a- 
further confirmation, he appealed to the stars. He observed 
Sirius, for instance, at a high altitude, and also at a lower 
altitude, and found that there was a darkening of some of 
the Fraunhoferic lines. Whether this was due to aqueous 
Saget cee BES <a 
rat 
ee eae 
J 
alata finlanditanking 
Fic, 17.—Angstrém’s map; showing telluric lines. 
vapour or to the air lines, of course could only be settled by 
reference to the absorption spectrum which he obtained from 
viewing a candle flame through the 330 feet of aqueous vapour, 
That distinguished man, Angstrém also examined into the sub- 
. Cee ee 7 = . . 
ject, and in his map are shown the lines which he considered to 
be due to the atmosphere, 
