150 REPORT—1868. 
objects, the correspondence between the results of prismatic and telescopic 
observation may be regarded as close and suggestive. 
Half of the nebulee - which give a continuous spectrum have been resolved, 
and about one-third more are probably resolvable; while of the gaseous 
nebule none have been certainly resolved, according to Lord Rosse. 
The inquiry now presses itself upon us—What superstructure of interpre- 
tation have we a right to raise upon the new facts with which the prism has 
furnished us ? 
Is the existence of the gaseous nebule an evidence of the reality of that 
primordial nebulous matter required by the theories of Sir William Herschel 
and Laplace? 
Again, if we do not accept the view that these nebule are composed of 
portions of the original elementary matter out of which suns and planets have 
been elaborated, what is the cosmical rank and relation which we ought to 
assign to them? As aids to a futwre determination of these great questions, 
some other observations made by the speaker may be briefly referred to, 
Comnts. 
There are objects in the heavens which occasionally, and under some con- 
ditions, resemble closely some of the nebule. In some positions in their 
orbits some of the comets appear as round vaporous masses, and except by 
their motion, cannot be distinguished from nebule. Does this occasional 
general resemblance indicate a similarity of nature? In 1864 Donati found 
that the spectrum of a comet visible in that year consisted of bright lines. 
Last January a small telescopic comet was visible. It was a nearly circular, 
very faint vaporous mass. Nearly in the centre, a small and rather dim 
nucleus was seen. When this object was viewed in the spectroscope, two 
spectra were distinguished. A very faint continuous spectrum of the coma 
showed that it was visible by reflecting the solar light. About the middle of 
this faint spectrum, a bright point was seen. This bright point is the spec- 
trum of the nucleus, and shows that its light is different from that of the 
coma. ‘This short bright line indicates that the nucleus of this comet was 
self-luminous ; and further, the position of this line of the spectrum suggests 
that the material of the comet might possibly be similar to the matter of 
which the gaseous nebulie consist. 
MEASURES OF THE INTRINSIC BRIGHTNESS OF THE NEBULZ. 
It appeared to the speaker that some information of the nature of the 
nebule might be obtained from observations of another order. If physical 
changes of the magnitude necessary for the conversion of these gaseous bodies 
into suns are now in progress in the nebule, surely this process of develop- 
ment would be accompanied by marked changes in the intrinsic brightness of 
their light, and in their size. 
Now since the spectroscope shows these bodies to be continuous masses of 
gas, it is possible to obtain an approximate measure of their real brightness. 
It is known that as long asa distant object remains of sensible size, its bright- 
ness remains unaltered. By a new photometric method the tntrinsic inten- 
sity of the light of three of the gaseous nebule was found in terms of a sperm 
candle burning at the rate of 158 grains per hour. 
Nebula No. 4628 ........ zen part of the intensity of the candle. 
Annular Nebula in Lyra .. 535, > Ps 3 
Dumb-bell Nebula ...... rotor KS 5 
0 39 
