AJs^D A SELECTION EEOM THE OBSEEVATIONS MADE WITH THEM. 703 
ance to the expression of opinion it conveys, because the question of resolvability can 
only be successfully investigated when the air is steady and the speculum in fine order. 
In the early observations with the 6-feet telescope we had the advantage of a very fine 
speculum ; it had been pohshed at the close of a long series of experiments with 3-feet 
specula, when by practice every refinement of manipulation was fresh in the recollec- 
tion ; there were also at that time several very good nights ; and many nebulas were 
resolved. Very soon after, the spu’al form of arrangement was detected; and our atten- 
tion was then directed to the form of nebulae, the question of resolvability being a 
secondary object. In the mean time the speculum, which had been frequently dewed 
and occasionally cleaned, had lost its fine edge, and was no longer in a state to deal with 
the question of resolvability. Our aim was to trace out faint details, and in that respect 
also the speculum had lost much of its power It was therefore repolished, and, though 
less perfect than before, did the work we required well. Since that, we have had per- 
haps two OT three specula as perfect as the first one ; but the mass of observations have 
been made with specula considerably inferior to it, and, I am sorry to say, very often not 
as bright as they should have been. The removing a 6-feet speculum from the tele- 
scope to the laboratory", repolishing it, perhaps several times, and replacing it, is a 
serious operation, and has often been too long postponed. While the telescope was in 
constant use in all weathers, it would have been a hopeless task to attempt to keep it 
in a state fit for the resolution of nebulae, and the attempt was not made. I may, 
perhaps, mention that with the 3-feet speculum in fine order I have often detected 
resolvability when there was no trace of it with the 6-feet speculum in its ordinary 
working-condition. 
The question of resolvability, therefore, I think, must remain to be taken up separately, 
when the finest instrumental means are available, and when it may no longer be neces- 
sary to subject the specula to the wear and tear of constant work. 
As to the nebulae which have nuclei, some are described as increasing in brightness 
very gradually to the centre, others very rapidly, and some as having a stellar nucleus, 
or perhaps a star in the centre. These descriptions, however accurately conveying the 
impressions made upon the eye at the time, cannot be taken as in all cases representing 
real physical facts. A star may have been mistaken for a condensed nucleus, or the 
reverse ; and it is often impossible to say which of the two suppositions is the more pro- 
bable. The remarks as to the question of resolvability apply with equal force to the 
questions relating to the structure of nuclei. It is, however, probably worth remarking, 
that, while amongst the clusters there are objects which, if removed to a sufficient 
distance, or examined with an instrument of insufficient power, may be supposed to be 
representations of nebulae with centres of varying brightness or condensation, there seems 
to be no cluster with a central star of such surpassing magnitude that under any circum- 
stances it could be taken as the representative of the class of objects described as having 
a star in the centre. 
5 c 
MDCCCLXI. 
