Mar. 27, 1873] 
a solid, a liquid or a dense gas or vapour. Again, suppose 
that instead of looking at the sun or the stars we observe 
the moon, as Fraiinhofer did, as has been ‘before stated, 
what will happen? We get a second edition of sunlight, 
in exactly the same way as we should get a second edition 
of the sunlight in the case of a reflection of it from a 
mirror; and therefore, if proof of such a thing were 
needed, the spectroscope is perfectly competent to show 
us that the moon gives us sunlight second-hand. The same 
in the main with Jupiter, Venus, Mars, and the other 
planets. If we study them and observe the dark lines we 
find that the lines which we observe are generally the same 
as those which we find in the spectrum of the sun. There 
are other points to which I shall have to draw your attention 
on a future occasion, but on the whole, the teaching of 
the spectroscope is, that all those planets are lit up by 
sunlight as we know them to be. 
But we have not yet exhausted the wonders of the 
celestial field ; we have dealt merely with the sun and 
moon, the stars and planets. What about the nebulz, 
those strange weird things, dimly shining in the depths 
of space, both to the eye and in the telescope obviously 
General view. 
Head and envelopes 
F:c. 38.—Views of Donati's Comet, 
and distinctly different from anything in the shape of the 
sun or stars? The appearance of these peculiar bodies 
is sufficient to show us that we have here something very 
different from the sun or moon. What is it? You all 
know as well as I do that ever since nebulx were dis- 
covered mankind have wondered at them, and wanted to 
know what they were ; and you are alsoaware that it was 
not settled and could not be settled before the advent of 
the spectroscope, but that it could be settled in five 
minutes after that event. Mr. Huggins, who first ob- 
served the spectrum of a nebula, found that, instead of 
the continuous spectrum with which you are familiar in 
the case of the sun and the stars—always asking you to 
neglect the Fraiinhofer lines, which I shall explain after- 
wards—the light which he got from the nebula consisted 
merely of three lines. He was exceedingly astonished, 
so much so that he thought the instrument might be out 
of order. However, it became perfectly clear to him in 
a very short time that there was no mistake at all, and that 
all that the light which came from the nebula could 
do was to give him these three faint lines. No doubt you 
have anticipated my explanation. The nebulz are com- 
posed of tenuous gases or vapours. After what I have 
said about the way in which the spectroscope at once 
picks out the difference between a solid or liquid, and a 
NATURE 
vaporous or a gaseous body, you will see at once that 
407 
these three bright lines indicate that the nebulz, instead 
of being composed of solid, liquid, or densely gaseous 
bodies—instead of being like the sun or stars—are really 
composed of rare gases or vapours. Mr. Huggins 
was enabled, in fact, to determine the gas in one 
instance, for one of the lines he found was coincident 
with one of the principal lines in the spectrum of hy- 
drogen one of the other lines possibly being due to 
nitrogen. And now comes another extremely important 
point, showing the importance of studying the most 
minute changes in gaseous spectra, for Mr. Huggins, 
who knew the spectrum of hydrogen and the spectrum of 
nitrogen well, and who knew how extremely complicated 
those spectra are at times, was much astonished at finding 
only one line of hydrogen and one of nitrogen, and 
attempted to account for the singleness of the lines, first, 
by assuming a condition of the gas different from anything 
Spectrum of Winnecke’s 
4, Spectrum of Carbon in 
a 
a 
o 
7 
“ 
i=} 
s 
2 
3 
3 
a 
rat 
oe 
S) 
= 
° 
5 
z- 
o 
v 
2. 
A 
(Huggins.) 
a 2 3 4 
we meet with in our laboratories, and again by assuming 
an absorbing medium in space, But after Dr. Frankland 
and myself had made some observations on the spectra 
of hydrogen and nitrogen, we found it was perfectly easy 
to obtain, and sometimes when one did not want it, a 
spectrum of hydrogen or of nitrogen giving only one line, 
or nearly so ; so that by comparing the conditions which 
were necessary to obtain these conditions in our tubes 
with the conditions of the nebulz, it was quite possible 
to make at all events a rough guess at what is the con- 
stitution of the nebulz, so far as pressure or molecular 
separation goes. We find, for instance, this single line of 
hydrogen, and a nearly single line of nitrogen, when the 
pressure is so slight that you would say that the tube 
really contained nothing at all, and when, moreover, the 
temperature is comparatively low. Now, not only is this 
a fact, which we are quite prepared to assert, merely on 
the evidence rendered us by these tubes, but I think you 
will acknowledge that it is entirely in accordance with 
everything we know astronomically on this subject. 
For the next application of the spectroscope in this 
direction, let us take a comet. The appearance of a 
