88 CELESTIAL SPECTROSCOPY. 
was suggested by Prof. Tait, and was brought to the notice of this As- 
sociation in 1871 by Sir William Thomson in his presidential address, 
The spectrum of the bright-line nebul is certainly not such a spee- 
trum as we should expect from the flashing by collisions of meteorites 
similar to those which have been analyzed in our laboratories. The 
strongest lines of the substances which in the case of such meteorites 
would first show themselves, iron, sodium, magnesium, nickel, ete., are 
not those which distinguish the nebular spectrum. On the contrary, 
this spectrum is chiefly remarkable for a few brilliant lines, very nar- 
row and defined, upon a background of a faint continuous spectrum, 
which contains numerous bright lines, and probably some lines of ab- 
sorption. 
The two most conspicuous lines have not been interpreted; for 
though the second line falls near, it is not coincident with a strong 
double line of iron. It is hardly necessary to say that though the near 
position of the brightest line to the bright double line of nitrogen, as 
seen in a small spectroscope in 1864, naturally suggested at that early 
time the possibility of the presence of this element in the nebule, I 
have been careful to point out, to prevent misapprehension, that in 
more recent years the nitrogen line and subsequently a lead line have 
been employed by me solely as fiducial points of reference in the spec- 
trum. 
The third line we know to be the second line of the first spectrum of 
hydrogen. Mr. Keeler has seen the first hydrogen line in the red, and 
photographs show that this hydrogen spectrum is probably present in 
its complete form, or nearly so, as we first learnt to know it in the ab- 
sorption spectrum of the white stars. 
We are not surprised to find associated with it the line D,, near the 
position of the absent sodium lines, probably due to the atom of some 
unknown gas, which in the sun can only show itself in the outbursts 
of highest temperature, and for this reason does not reveal itself by 
absorption in the solar spectrum. 
It is not unreasonable to assume that the two brightest lines, which 
are of the same order, are produced by substances of a similar nature, 
in which a vibratory motion corresponding to a very high temperature 
is also necessary. These substances, as well as that represented by 
the line D3, may be possibly some of the unknown elements which are 
wanting in our terrestrial chemistry between hydrogen and lithium, 
unless indeed D, be on the lighter side of hydrogen. 
In the laboratory we must have recourse to the electric discharge to 
bring out the spectrum of hydrogen; but in a vacuum tube, though 
the radiation may be great, from the relative fewness of the luminous 
atoms or molecules or from some other cause, the temperature of the 
gas as a whole may be low. 
On account of the large extent of the nebule, a comparatively smal 
number of luminous molecules or atoms would probably be sufficient 
