. 
lope which I have called the Chromosphcre, 
*y ioeieuen . e 
: - Pe 
Fune 5, 1873] _ NATURE 105 
tant fact connected with these lines, which appear | are travelling over the faz» «f «re central Jumi- 
when magnesium vapour is thrown up into the enve- 
By 
means of the new method of research, it is quite 
“possible to see, as I explained to you on a former occasion, 
what passes, which the eye could not possibly see. For 
instance, it is quite possible, by means of the spectro- 
scope, to detect the existence of magnesium vapour out- 
side the sun, although you know that, except during 
eclipses, we are never able to see these vapours. What 
I wish to call your attention to in the present case is this. 
We have there the three magnesium lines, and two of 
them are much thicker than the remaining one: and 
these two lines travel very much higher into the outside 
region than does the third cne. Now, ycu will see ina 
moment that that indicates to usa fact something like 
this,—that the spectrum of magnesium, such as is gene- 
rally at work, which cuts out these very black absorption 
lines in the solar spectrum. while the sodium gives us 
the yellow line D, is really a thing which is competent to 
give us three lines. This vapcur, I say, is a thing, gene- 
rally speaking, competent to give us three lines in this 
position ; but if it so happens that when the magnesium 
is thrown up to a particular height we simply get two 
lines, the third stopping short, I think you will see that 
there is some force in one’s reasoning, when one suggests 
that possibly in those regions where we find the hydrogen 
F line thin instead of thick, as I have shown it to you, 
and where the magnesium lines become reduced to two 
instead of three, the spectrum of magnesium vapour, like 
the spectrum of hydrogen, becomes very much more 
simple by the reduction of pressure, and therefore, that we 
should be able artificially, as in the case of hydrogen, and 
as inthe case of sodium, to reproduce this result. In 
fact, it is perfectly easy to reproduce it, for we find by 
reducing the pressure of magnesium vapour we really can 
reduce that triple line of magnesium to a double one ; so 
that, you see, we have three distinct lines of research, all 
leading us to the fact that where Kirchhoff placed an im- 
mensely dense atmosphere around a liquid sun, we really 
have vapour of considerable tenuity, by no means so 
dense as he supposed. 
There is another point of very great interest which I 
should bring before you. ; 
Mr. Huggins, who has done so much in his researches 
on stars, told us some few years ago that the spectrum 
of that wonderful variable star + Coronz, which had 
been just discovered, indicated that, over and above 
the light which we got from the star generally, we 
get evidence of incandesccnt hydrogen in the spec- 
trum, so that the spectrum was a thing such as had 
never been seen before; for we got, in addition to 
the ordinary evidence of absorption visible in the 
spectrum of a star, as in the spectrum of the sun, indi- 
cations also of selective radiation. There are indications 
of beght lines superposed above the others. Now, let 
me tell you—and this is a very important part of the ques- 
tion—that by observing the various changes that take 
place in our central luminary, it is quite possible to see on 
the sun almost any day evidence of its being violently 
agitated ; that there are certain regions of the sun which 
appear exactly as that variable star did—that is to say, 
in addition to the ordinary absorption lines visible in the 
solar spectrum, the spectrum of these regions indicates to 
us that the hydrogen, instead of being black, instead of 
reversing the spectrum, as you have seen it in these 
spectra that I have shown you, really is bright, or else 
e hydrogen lines cease to be visible altogether, as in 
a Orionis. 
I have to give you, as the last application of spectrum 
analysis, the power which the prism gives us of inves- 
tigating, so to speak, the meteorology of the sun, the 
velocity with which the different stars are moving 
through space, and the velocity with which the storms 
nary. Many of ycu know, no doubt, that Mr. Huggins, 
in his observations of the spectrum of the star Sirius, 
saw that the hydrogen lines were much developed ; and 
in a further examination, carried on by the method in 
which the spectrum of hydrogen and other vapours which 
he wished to examine were absolutely visible in the field 
of view at the same time as was the spectrum of the star, 
Mr. Huggins was astonished to find that the hydrogen 
lines no longer occupied their usual positions, but that 
they were all jerked, so to speak, a little to the side of 
the place which they occupied in the spectrum of the 
hydrogen which he rendered incandescent in his tubes. 
The F line of hydrogen which he observed in the spec- 
trum of Sirius he found did not exactly occupy the 
same position in the spectrum as did the actual F line 
of hydregen, the incandescent hydrogen with which he 
compared it (Fig. 53). Owing to a physical law, which I 
have not time to explain to you now, it is perfectly easy, by 
means of the prism, to determine the velocity with which 
the light-source is moving to or from us; and therefore, 
if this holds good for absorption, we could determine the 
velocity with which any absorbing medium is rushing to 
or receding from us. In the case of Sirius, for instance, 
Mr. Huggins determined that the velocity of the star in 
a direction from the eye, the measure of recession, was 
something like twenty miles a second. I am sorryI have 
not time to fully explain this very beautiful adaptation of 
the spectroscope, but I may say that the position of a 
line, bright or dark, in the spectrum depends upon its 
wave-length—that is to say, the length of the wave of 
“light which produces that colour. Thus, the length of a- 
wave of red light is about yy}o99 of an inch, and that ofa 
wave of violet light is about ; 155 of an inch. I think 
when I mention that, you will see at once the possibility 
of determining any alteration of velocity—for an altera- 
tion of wave velocity we have, or appear to have, whether 
we move towards an object, or whether an object moves 
towards us, just in the same way as in the case of sound, 
and in the case of a wave reaching the shore. Suppose 
yourself a swimmer carried on a wave; if you are going 
with the wave it seems long, but if you attempt to swim 
against it it seems short. So with all these waves, beating 
from all these orbs peopling the depths of space on to the 
earth. If by the motion of those bodies or by our own mo- 
tion, the waves are crushed together, we get an alteration in 
the light, which the prism alone is able to determine. If 
the luminous object is approaching the eye rapidly, the 
vibrations causing light will, of course, fall on the eye 
more frequently in the same time than if the bodies were 
at rest—or, in other words, the waves will be shortened ; 
then the position of the dark or bright lines, as the case 
may be, will be shifted in the direction of the most re- 
frangible rays—that is to say, towards the violet ; whilst 
ifthe bodies are separating, the shifting will take place in 
the direction of the red or least refrangible rays. In the 
case of Sirius, the star was receding from us, and we got 
longer waves, and the lines are nearer the red end of 
the spectrum to such an extent as to leave unaccounted 
for a motion of recession from our sun amounting to some- 
thing between 18and 22 miles per second. Other stars, such 
as Betelgeux, Rigel, Castor, Regulus, and many of the stars 
in Ursa Major, are found to be moving away from the sun. 
Some, however, move rapidly towards us. Arcturus ap- 
proaches us with a velocity of 55 miles per second ; 
Vega and a Cygni, Pollux and @ Ursa Majoris, also ap- 
proach the sun with a velocity varying from 40 to 60 
miles per second. If now we take a spot-spectrum 
(Fig. 54), in which, instead of the sodium line D, we 
have the F line of hydrogen, this strange crookedness 
which you notice is really a crookedness due to the fact 
that in one place we have incandescent hydrogen rising 
up with tremendous velocity, and in another we have it 
rushing down cool with tremendous velocity ; again, we 
