the red, marked C, which is absolutely coincident with 
a prominent dark line in the solar spectrum. Now 
this is a black line which, by repeated observations, we 
know corresponds in degree of refrangibility exactly 
with one of the lines given out by glowing hydrozen, 
when examined in one of these tubes with the electric 
spark. When, therefore, we get any substance around 
the sun reporting its light to us, it is perfectly obvious, I 
think, that if the bright line really be coincident with this 
dark line, that something is probably hydrogen. This 
was one of the first lines determined by M. Janssen 
in the eclipse of 1868. There is another bright line 
absolutely coincident with a dark line known to corre- 
spond in refrangibility with another line given out by 
| DARK BAND 
Wis, : 
sure of these circum-solar regions which the spectroscope 
has determined to be occupied by an envelope of hydro- 
gen gas, mingled sometimes with other vapours, which 
envelope I have termed the chromosphere. When the 
pressure of the chromosphere is completely determined, 
we shall be probably enabled to determine the tempera- 
ture of the sun. 
A line again in the violet corresponds with a dark 
line in the solar spectrum, which is coincident with a 
third line of glowing hydrogen which we have before 
spoken about, and there is still another coincident 
line. A line in the yellow of the spectrum will also 
be noticed. This is one which has caused a great 
deal of discussion, for it is not coincident with any 
line of any known terrestrial substance. A number o! 
short lines are also shown in the engraving which will be 
seen to correspond to the part of the chromosphere which 
is denser, for then the F line of hydrogen has become 
broad where these lines are seen ; these lines show that 
_ in the layers of the chromosphere nearest to the sun a 
number of other substances exist, amongst which may be 
mentioned magnesium, iron, and sodium, The reason 
that bodies do not reach up so far from the body of the sun 
is that their vapours are very much heavier than the gas 
hydrogen, which is the lightest terrestrial substance known. 
Such are a few of the practical applications of the spec- 
troscope as applied to the radiation of light. There are 
other classes of facts relating to the absorption of light, 
on the consideration of which we shall now enter. 
The subject with which we have just been dealing 
is the radiation or giving out of light by bodies in 
different states—that is to say, by solid or liquid bodies, 
or gaseous or vaporous ones. We have now to deal with 
the action of the prism upon light under some new con- 
ditions—conditions which I purposely withheld from you 
in the last lecture. Light is not only given out, or vadi- 
ated, but it may be stopped or adsorbed in its passage 
from the light-source to our eye, if we interpose in the 
path of the beam certain more or less perfectly trans- 
parent substances, be they solids, liquids, gases, or 
vapours. I will recall one or two of the experiments 
which have been already described in order that you 
ate ee rr onl ‘elke CP oe ae Pay ™* 
? J a nae . é ¥ * 
April 17, 1873] NATURE 467 
hydrogen in the green part of the spectrum, marked F in 
the figure. This, then, is further proof in favour of hy- 
drogen ; and now notice a great difference between the 
shape of this line and the red line which I drew your 
attention to just now. An enlarged representation of this 
line is shown in Fig. 42. 
You will bear in mind what I told you about the effect of 
pressure in altering the spectrum of hydrogen, and that 
one of the most obvious effects of increased pressure was 
to increase the thickness of what is called the F line—the 
line now under consideration, you will see here that the 
widening of the F line, the green line of hydrogen, really 
indicates a thickening due to pressure. In that way 
we have been able to determine approximately the pres- 
/N MAGENTA. 
a 
| iil 
a 
may see exactly how the perfectly distinct classes of 
phenomena due to radiation and absorption really run 
together. You will recollect that I pointed out to you 
that radiation, or the giving out of light, might be con- 
tinuous or might be selective, and I am anxious now to 
show you that radiation is exactly equalled by absorption 
in this matter; that absorption may also be continuous 
or selective. We have before taken as an instance of 
continuous radiation a continuous spectrum obtained by 
using the electric lamp or a lime-light ; that is to say, an 
example of the general radiation which you get from an 
incandescent solid—the carbon points of which the poles 
Fic. 44.—Method of observing the absorption of a vapour. 
of the lamp are composed, or the solid lime. You will 
remember that if we take the spectrum of a vapour—as, 
for instance, that of strontium or thallium—we find that 
the continuous spectrum is altogether changed, and that 
in the place of that beautiful rainbow band, continuous 
from the red end of the spectrum to the violet, we really 
only get lines here and there, which are due to the se- 
lective radiation, and opposed to the general radiation 
which we spoke of in the continuous spectrum just now. 
I might have chosen other substances besides strontium 
and thallium, but I mentioned the spectra of these sub- 
stances when we were considering the question of radia- 
