126 Royal Society. 



Clusters of Stars, made at Slough, with a Twenty-feet Reflector, be- 

 tween the Years 1825 and 1833." By Sir John F. W. Herschel, 

 K.H., F.R.S. 



This paper contains the results of observations begun in 1825, and 

 assiduously prosecuted till the commencement of the present year, 

 for the purpose of reviewing the nebulae and clusters of stars disco- 

 vered by his father, the late Sir William Herschel, and also of extend- 

 ing his discoveries, and enlarging our knowledge of the nature and 

 physical constitution of those remarkable and mysterious bodies. 

 Since the recent improvements in the achromatic telescope, and the 

 increased diligence of astronomers in surveying every part of the 

 heavens, and detecting the passage of comets, the want of an exten- 

 sive list of nebulae has become continually more urgent ; and hence 

 the author was induced to supply, as far as he was able, that deficiency, 

 which he has now attempted by simply stating the results of his own 

 observations, in preference to waiting until he could present them to 

 the Society in the more complete form of a general catalogue of ne- 

 bulae and clusters visible in this latitude. All the observations here 

 given have been reduced to a common epoch, and arranged in the 

 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 nebulae 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 nebulae 

 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 fire rcprcsenta- 



