214 



order of right ascension : and in every case where the same object 

 was observed more than once, all the observations relating to it have 

 been collected together ; by which means they not only can be used 

 as a catalogue for reference, but each result carries with it its own 

 weight and evidence. 



Great and various are the difficulties attending inquiries of this 

 nature. Many of the nebulas present a surface so large and ill de- 

 fined, that it is not always easy to determine where the centre of 

 greatest brightness is situated. Vast numbers of the nebulae, indeed, 

 are so extremely faint, as to be with difficulty perceived, till they 

 have been some time in the field of vision, or are even just about to 

 quit it j so that the observations become hurried and uncertain. In 

 those parts of the heavens where they are most crowded, their pro- 

 digious number, as well as their variety, and the interest they excite, 

 render it scarcely possible to proceed with that methodical calmness 

 and regularity which are necessary to ensure numerical correctness. 

 It is also to be recollected, that it is only during the months of 

 March, April, and May, that the richer parts of the heavens can be 

 advantageously observed, and then only in the complete absence of 

 the moon and of twilight. From all these causes conjoined, it will be 

 readily understood, that a much greater latitude of error is incident 

 to observations of nebulae than to those of stars. 



The observations registered in this paper comprise 2500 nebulas 

 and clusters of stars, — a number equal to that of those observed by 

 Sir William Herschel : only about 2000, however, are common to 

 both collections, the remaining 500 of the author's being new. Of 

 these last, by far the greater proportion are objects of the last degree 

 of faintness, only to be seen with much attention, and in good states 

 both of the atmosphere and the telescope. The author generally 

 made a sketch of any remarkable nebula that presented itself ; and 

 these drawings accompany the paper. Among these are representa- 

 tions of some very extraordinary objects,, which have not hitherto suf- 

 ficiently engaged the attention of astronomers, and many of which 

 possess a symmetry of parts, and a unity of design, strongly marking 

 them as systems of a definite nature, each complete in itself, and sub- 

 servient to some distinct, though to us inscrutable, purpose. 



In an Appendix, the author enters into a detailed account of the 

 manner in which the reductions have been executed, and how the 

 numbers set down in the catalogue are concluded from those regi- 

 stered at the moment of observation. For effecting these reductions, 

 he pursued a method materially different, and much more convenient 

 and exact, than he employed to reduce his earlier catalogue of double 

 stars. 



Various remarks are next made on the figured nebulae. It often 

 occurred to the author, to notice a peculiar state of the atmosphere, 

 which is quite independent of fog or haziness, in which all large stars 

 above the seventh magnitude appear surrounded with photospheres, 

 Of a diameter of two or three minutes, or even more, and exactly re- 

 sembling those about some of the finer specimens of nebulous stars. 

 These appearances come on suddenly, seldom last long, and disap- 



