2 



Herman Schultz, 



An important point in regard to nebular science , but which , among the 

 many other interesting questions that here present themselves, has not hi- 

 therto received sufficient attention, is the question of the proper motion of 

 the nebulae, and the principal object of the present work is to offer a con- 

 tribution to that particular branch of research in the nebular-world. The 

 question of the proper motion of nebulas and telescopic clusters, which must 

 necessarily be preparatory to that of the dynamical relations within the clus- 

 ters and between the nebulae and the starry universe in a restricted sense, 

 is without doubt one of the most important in the present state of astro- 

 nomical science; and the demand for a body of observations, which it is 

 our duty to leave to posterity, appears in this case to be inignorable, 

 although the study in question is as yet so little advanced that we can 

 hardly hope in our own time to obtain any important results from our ob- 

 servations. 



The question of the nebulae's proper motion had indeed been early 

 raised, and the determinations made of their positions were, some at least 

 of them, intended tor the future solution of the questions relative to this 

 subject. It seems however to be beyond a doubt, that, with the exception 

 of the positions of some few more brilliant nebulae, those, which were de- 

 termined before Laugier's observation at Paris and those af D'Arrest »t 

 Leipzig, are too deficient in accuracy for it to be probable, that they 

 can be of any real use for that purpose. It is also a well known fact, 

 that the above mentioned series of observations were the first that were 

 undertaken exclusively for the purpose here referred to, whereas the chief 

 object as well of Sir W. and Sir J. Herschel's, Lord Rosse's and Lassel's 

 work, as of D'Arrest's later observations, was, as has been already intimated, 

 such as did not require the greatest possible precision in the positions. The 

 uncertainty in W. Herschel's positions is never less than 1' or 2'. Al- 

 ready those of J. Herschel are far more accurate, inasmuch as that irre- 

 spective of confusions and incidental errors, the probable errors in a posi- 

 tion do not, according to D'Arrest, exceed 1' and 20". D'Arrest's Co- 

 penhagen-observations, approaching in form nearly those of Sir J. Herschel, 

 yet are superior to them as regards the absence of incident errors and con- 

 fusions; but even here, according to the author's own statement, the probable 

 errors in a position amount to about 0, s 8 and 18". But although the right 

 ascensions and declinations given in all these series of observations do not 

 possess sufficient accuracy for the purpose that we have before us, yet we 

 must not overlook the fact, that in many instances these catalogues contain 

 differential determinations of the positions of nebulae referred to neighbouring 



