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XII. Observations on some of the Nebulce. By the Earl o/’Rosse, F.R. S., «fc. 
Received June 10, — Read June 13, 1844. 
As every addition, however trifling, to the little we know with certainty respecting 
the nebulae can scarcely be considered wholly uninteresting, I have ventured to com- 
municate a few observations made with the speculum of three feet aperture described 
in the Philosophical Transactions for 1840. 
In using that instrument, the object was rather to test its powers, and to decide the 
merits of progressive experiments, than to seek for astronomical results, and there- 
fore the micrometer w r as not employed; in every case, however, the sketches were 
repeatedly compared with the originals, and having usually the advantage of the 
opinion of one or more friends, and always of that of my assistant, I believe they will 
be found to be tolerably correct. Without accurate micrometrical measurements 
any sketch can be of comparatively little value as an astronomical record, since it 
would be scarcely safe under any circumstances to consider it as decisive evidence, 
where the question was whether any change had or had not taken place in the general 
outline or internal structure of a nebula, or in the relative positions of the remarkable 
stars in or near it. Still measurements to be valuable must be exact ; and it would 
perhaps have been misspent time to have employed the micrometer with an instrument 
not well suited to the purpose, when there was a prospect of being under the necessity 
of repeating the same operations again, probably at a very short interval, in making 
a complete examination of the nebulse with the instruments now in progress. 
From these trifling sketches, however, we may perhaps faintly see some indications 
of the course which our speculations on the physical structure of the nebulse are 
likely to take under the guidance of increasing information. Some estimate may 
also perhaps be made of the amount of knowledge to be gained by an examination of 
all the known nebulse with instruments like the present telescope, which astronomers 
favourably circumstanced may construct without any very serious difficulty. 
The actual time in one year during which a powerful telescope can be used effec- 
tively is so short, that where observations must be accompanied by sketches, the 
progress is necessarily slow; and it is still more so when the micrometrical measure- 
ment of faint objects becomes an essential portion of the work. Out of a consider- 
able number of tolerably good working nights there are very few, and even then 
often but for a short time, when high magnifying powers can be employed; so that 
upon the whole a great deal cannot be accomplished by one instrument in a limited 
period. As to the present telescope, it has not been constantly employed. Unless 
the beginning of the night was favourable nothing was ever done ; a regular system 
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