45] ROYAL ASTRONOMICAL SOCIETY TO PROF. H. D'ARREST. 351 



having stellar nuclei, or at least an entirely regular form, were observed 

 with all possible precision, and the differential determinations of their positions 

 referred to neighbouring stars will, without doubt, be found of the greatest 

 importance in the future study of their proper motions. 



Excluding a few nebulae, whose places do not admit of any accurate 

 determination, Professor D'Arrest finds, from 1627 observations of declination 

 of 525 nebulae, that the probable error of a single observation of declination 

 is 17"'58, while from 1552 right ascension observations of 497 nebulae, he 

 finds the probable error of a single observation of right ascension to be 

 0"-809 sec S. 



These probable errors are slightly less than the corresponding probable 

 errors of Sir John Herschel's catalogues. 



Following the excellent example set by Sir John Herschel, Professor 

 D'Arrest gives the results of each night's observations of a nebula separately, 

 both as regards its place and its description. 



The use of an equatorially- mounted telescope has no doubt rendered 

 this catalogue comparatively free from incidental errors and mistakes in the 

 identification of nebulae, which will occasionally happen, in spite of the 

 greatest care, when the observations are made with an instrument not so 

 mounted. 



Lord Ptosse's valuable selection from the observations of nebulas made 

 with his gigantic reflector of 6-feet aperture appeared in the Philosophical 

 Transactions for 1861, but, curiously enough, did not reach Professor D' Arrest's 

 hands till 1864, when his own work was considerably advanced. This work 

 contains sometimes brief and sometimes full descriptions of about 800 nebulae, 

 many of them being illustrated by figures. Professor D'Arrest found that 

 not a fe\y of the nebulae which he had detected in the interval between 

 1861 and 1864 had been already observed by Lord Kosse and his assistants, 

 and that his descriptions were generally confirmed by theirs. Very^ many 

 " new " nebulae, however, still remained which had not been observed by 

 Lord Rosse; while, on the other hand, many which occur in Lord Rosse's 

 work had escaped the notice of Professor D'Arrest. After this period he 

 derived the greatest assistance from Lord Rosse's work. It is not surprising 

 to find occasional differences and discrepancies in the descriptions of nebulae 

 given in these two works. Professor D'Arrest mentions that he has found 

 and observed by far the greater part of those nebulae which had been 



