OF SOME OF THE STAES AND NEBULA. 
541 
The faint continuous spectrum, which in some cases is also seen, has been traced in 
certain nebulse, by its breadth, to a distinct brighter portion of the nebula which it is 
convenient still to distinguish by the term “nucleus,” though at present we know 
nothing of the true relation of the bright points of the nebulse to the more diffused 
surrounding portions. 
It must not be forgotten that when gases are rendered luminous there may usually be 
detected a faintly luminous continuous spectrum. In the case of several of the nebulse, 
such as the annular nebula of Lyra and the Dumb-bell nebula, no existence of even a 
faint continuous spectrum has been yet certainly detected. 
The determination of the position in the spectrum of the three bright lines was 
obtained by simultaneous comparison with the lines of hydrogen, nitrogen, and barium. 
The instrument which I employed had two prisms, each with a refracting angle of 60°, 
and the positions of the lines were trustworthy within the limits of about the breadth 
of the double line D. 
The objects which I proposed to myself, in attempting a reexamination of some of the 
nebulae with the large instrument described in this paper, were to determine, first, 
whether any of the nebulae were possessed of a motion which could be detected by a 
change of refrangibility ; secondly, whether the coincidence which had been observed of 
the first and the third line with a line of hydrogen and a line of nitrogen would be found 
to hold good when subjected to the test of a spreading out of the spectrum three or 
four times greater than that under which the former observations were made. It would 
not, it seemed, be difficult, in the case of the detection of a want of coincidence, 
to separate the effects of the two distinct sources referred to, from both of which equally 
a minute difference of refrangibility between the nebular lines and those of terrestrial 
substances might arise. The probability is very great indeed that in all the nebulae 
which give the kind of spectrum of which I am speaking, the two lines referred to are 
to be attributed to the same two substances, and that therefore, in all these nebulae, 
they were originally of the same degree of refrangibility. On the other hand, it is not 
to be supposed that nebulae situated in different positions in the heavens would have a 
similar motion relatively to the earth. An examination of several nebulae would 
therefore show to which of these causes any observed want of coincidence was to be 
attributed. 
The Great Nebula in Orion . — In my description of this nebula* I stated that the light 
from all the parts of this strangely diversified object, which were bright enough to be 
observed with my instrument, was resolved into three bright lines similar to those 
represented in the diagram. 
On the present occasion I applied myself in the first place to as careful a comparison 
as possible of the brightest line with the corresponding line of the spectrum of nitrogen. 
My first observations were made with the light from the induction-spark taken in pure 
nitrogen sealed in a tube at a tension a little less than that of the atmosphere, whicli 
* Proceedings of the Eoyal Society, vol. xiv. p. 39. 
