148 REPORT—1868. 
three isolated bright lines only. The spectrum of this nebula is represented 
in fig. 5 of Plate ILI. 
This observation was sufficient to solve the long-agitated inquiry in refer- 
ence to this object at least, and to show that it was not a group of stars, but 
a true nebula. 
A spectrum of this character, so far as our knowledge at present extends, 
can be produced only by light which has emanated from matter in the state 
of gas. The light of this nebula, therefore, was not emitted from incan- 
descent solid or liquid matter, as is the light of the sun and stars, but from 
glowing or luminous gas. 
It was of importance to learn, if possible, from the position of these 
bright lines, the chemical nature of the gas or gases of which this nebula 
consists. 
Measures taken by the micrometer of the most brilliant of the bright 
lines showed that this line occurs in the spectrum very nearly in the position 
of the brightest of the lines in spectrum of nitrogen. The experiment was 
then made of comparing the spectrum of nitrogen directly with the bright 
lines of the nebule. The speaker found that the brightest of the lines of 
the nebulse coincided with the strongest of the group of lines which are 
peculiar to nitrogen. It may be, therefore, that the occurrence of this one 
line only indicates a form of matter more elementary than nitrogen, and 
which our analysis has not yet enabled us to detect. 
In a similar manner the faintest of the lines was found to coincide with 
the line of hydrogen coincident with F. 
The middle line of the three lines which form the spectrum of the nebula 
does not coincide with any very strong line in the spectra of about thirty of 
the terrestrial elements. It is not far from a line of barium, but it does 
not coincide with it. Besides these bright lines there was also an exceedingly 
faint continuous spectrum. The spectrum had no apparent breadth, and 
must therefore have been formed by a minute point of light. Its position, 
crossing the bright line about the middle, showed that the point of light was 
situated about the centre of the nebula. Now this nebula possesses a mi- 
nute but bright nucleus. We learn from this observation that the matter 
of the nucleus is almost certainly not in a state of gas, as is the material of 
the surrounding nebula. It consists of opake matter, which may exist in 
the form of an incandescent fog of solid or liquid particles. 
The new and unexpected results arrived at by the prismatic examination 
of this nebula showed the importance of examining as many as possible of 
these remarkable bodies. Would all the nebule give similar spectra? Espe- 
cially it was of importance to ascertain whether those nebule which the 
telescope had certainly resolved into a close aggregation of bright points 
would give a spectrum indicating gaseity. 
The observation with the prism of these objects is extremely difficult, on 
account of their great faintness. Besides this, it is only when the sky is 
very clear and the moon is absent that the prismatic examination of their 
light is even possible. During the last two years the speaker has examined 
the spectra of more than 60 nebule and clusters. These may be divided into 
two great groups. One group consists of the nebule which give a spec- 
trum similar to the one already described, or else of one or two only of the 
three bright lines. Of the 60 objects examined about one-third belong to 
the class of gaseous bodies. The light from the remaining forty nebula and 
clusters becomes spread out by the prism into a spectrum which j is apparently 
continuous, 
