206 
NATURE 
[Fuly t, 1886 
instead of appearing to us with a disk it should appear to usas a 
star, like Sirius or Capella, for instance—the only difference 
between its spectrum now and its spectrum then would be that 
there would be less of it. There would be less light. Conse- 
quently it would not be possible for us to see it in all its exquisite 
detail. But so far as the spectrum went there would be no 
change in kind, although there might be a change in degree. 
Now, if you just assume that for a moment, you see that we 
shall be in a very fair way tomake a very important application 
of this knowledge, because [ was careful to tell you that in the 
solar system we have indications of a considerable amount of 
absorption of blue light ; so that, if the sun’s atmosphere were 
away and the earth’s atmosphere were away, the sunlight, if we 
are now right in calling it white, would then certainly appear to 
us as blue, for the reason that the blue light now stopped by the 
sun’s atmosphere and by our own would then be added to the 
light which we get at the present moment, and the total light 
therefore received by our eyes would be very much richer in 
blue rays than it is at present. 
Now then, having the fact of this blue absorption in our minds, 
let us suppose it—to begin with the simplest case—to be enor- 
mously increased. Let the blue absorption creep on into the 
spectrum fill at last it reaches the green or the yellow or the red. 
It is clear that then the sun that we should see would be a red 
sun, and that sunlight would be no longer white, but red. 
Let us next, on the other hand, reduce the quantity of the 
existing blue absorption. Let us have a solar spectrum as long 
as the spectrum of the electric light, for instance. 
Now let us do something else. Let us suppose that in the 
solar spectrum, as in very many of the spectra that we can ob- 
serve in our laboratories, there is superadded to this blue absorp- 
tion a strong absorption of the red, beginning at the other 
end of the spectrum. We shall get the yellow and the 
red, say, absorbed on the one side of the spectrum, while we get 
the blue and violet absorbed on the other. We shall therefore 
only get the green light to pass. 
Do we get evidence that in the heavens among other 
stars such conditions as these hold? Certainly. A very 
considerable number of the stars in the heavens are called 
coloured stars. They are red, or they are blue, or they 
are green, for the most part, and you see that simply dealing with 
the absorption of the blue with which we have become familiar 
in the case of the sun, playing with it a little, giving it a little 
rope here, shortening the rope there, and adding another exactly 
equivalent absorption at the other end of the spectrum, we can 
at once account simply and sufficiently for the colours of the 
coloured stars. This is one advantage that we have in 
working from the known to the unknown. If we had begun 
with the stars and dealt with their phenomena first, it would have 
been difficult to explain; but now that we know how a thing 
happens in the case of the sun, it is quite easy for us to imagine 
the mechanism which must be at work in the atmosphere of the 
coloured stars to give us in some cases red suns, in others green 
suns, and in others still blue suns. 
So much then for coloured stars. 
There is another matter. As I shall have to show you by and 
by, one of the most important distinctions between the stars in 
the heavens is one not depending upon their magnitudes, not 
depending upon their distances, or upon their mass, or upon any- 
thing of that kind, but depending upon conditions which we do 
not know very much about at present, but which bring about this 
result, that the spectrum in one case is different from the spectrum 
in another, exactly as in our laboratories we find the spectra of 
bodies with which we are perfectly acquainted become different 
if the temperature which we employ is made to differ. For in- 
stance, in the case of the vapour of carbon we may employ a low 
temperature, and get a certain spectrum of the vapour which is 
called a spectrum of flutings. If we increase the temperature, 
and then again observe, the flutings have disappeared. They 
have given way to a system of lines in which the irregularity 
is just as striking as the exquisite rhythm of the flutings 
was in the former case. From hundreds of these observations 
the student of spectrum analysis is not afraid to say that when he 
‘sees a spectrum of flutings he knows that he is dealing with the 
action of vapours at a much lower temperature than exists in 
those conditions in which the flutings are replaced by lines. 
And, more than that, so definite is this, so much do we know 
about the fluted spectra of those substances which exist in the 
solar atmosphere—giving us, at the temperature of the sun, the 
dine spectrum—that it is easy for us to take the responsibility also 
| reasoning is right. 
of saying that, if the sun’s atmosphere were to be suddenly cooled 
to-morrow, we should get a spectrum of flutings, instead of a 
spectrum of lines ; so that when we get, if we do get, the fluted 
spectrum in the spectrum of a star, we are justified in saying that 
some cause has been at work in that star equivalent to a cooling 
process in the atmosphere of our own star. Thus, if we cooled 
the sun to-morrow we should produce the spectrum of flutings, 
and as in cooling down the sun will in all probability pass through 
a stage indicated by flutings, so also while it was acquiring its 
present temperature it passed through the same stage. 
What, on the other hand, would happen if we had the sun 
very much hotter to-morrow? It is important to think this out 
very carefully. According to the views which I have brought 
before you, we have, outside all, solids absorbing every part of the 
spectrum. Then we have liquids and dense vapours doing the 
same: less dense vapours absorbing the red, and finer vapours 
still absorbing the blue. We have flutings also, but chiefly 
we have vapours at an enormous temperature which give us the 
familiar absorption spectrum of Fraunhofer lines. 
We have the Fraunhofer spectrum in short giving us the sum- 
mation of the line absorption of every stratum in the sun’s 
atmosphere. We have also a wonderfully simple spectrum of 
the chromosphere, of which I gave you the list of lines, 
writing down for us the absorption of the hottest part of the 
sun’s atmosphere that we can get at. 
Now try to think this out quite completely. 
The first obvious thing which will strike us is that, if the sun 
could be made hotter to morrow than it is to-day, the thing that 
we should be quite certain about, whatever might happen to the 
other conditions, would be that the gases which give us that 
simple spectrum of the chromosphere would have a larger share 
in the absorption-spectrum, and that therefore the absorption- 
spectrum of the star would gradually get nearer and nearer to the 
absorption-spectrum which would be given by the chromosphere 
itself if it could be seen in all its simplicity. I think that way of 
Well, if you think it is, you will find that it 
will lead us to a very interesting conclusion. If we find any star 
with practically the spectrum of the chromosphere, we shall be 
bound to admit that the atmosphere of that star must be hotter 
than the average temperature of the atmosphere of our sun as its 
spectrum approaches that of the Aoé/est part of the sun’s atmo- 
sphere. 
There is one other point that I have to bring before you 
before I go further, and it is this. We have had a great deal 
to say about the photosphere of the sun and the surrounding 
envelopes. We saw that when any vapours were Iccated be- 
tween our eye and the bright sun in the centre we then got 
absorption-lines, for the reason that the sun was hotter than the 
vapour on this side of the sun, so to speak, and therefore light 
was stopped by the cooler vapour in the atmosphere, and we got 
a dark line. The moment however, we work outside the disk, 
and study a prominence on the limb of the sun, or even a part 
of the corona, we observe them by means of their bright lines— 
by means of their radiation. ‘There is no hotter light source 
behind them, and therefore we deal simply with radiation. 
Now, that being so, you will understand how it is that in the 
general spectrum of the sun all the lines are dark, because we 
found that while the bright central part of the sun was not very 
much less than the whole volume, something like a tenth, it was 
very much hotter, so that we get many thousand times more light 
from the centre of the sun. If a substance in the outer atmo- 
sphere gives us a bright line corresponding with a dark line 
given us from this central portion due to the atmospheric ab- 
sorption, all it can do is to reduce the intensity of the dark line 
produced by the intensely illuminated central portion. 
It is a question of area. The difference of area is small, 
smaller than the difference of illumination, and therefore any- 
thing which happens outside does not get its record written at 
all, the area being five or six to one, and the intensity of the 
light in the centre being, say, ten thousand to one. 
Now let us consider another case. Let us suppose that there 
s a star (never mind which it is) the atmosphere of which is so 
enormous that its diameter to the diameter of the central photo- 
sphere is represented by two concentric circles—one very large, 
the other very small. Here the difference of area between 
the inner circle, which gives us dark lines, and the larger exte- 
rior space, which gives us bright lines, if it gives us anything, 
is so enormous that it may be greater than the difference of 
the intensity of the light ; so that if the inner light is ten 
times brighter than the light which comes from the outer 
e-eupmeeten > es Cy ew ee 
