Observations of 500 Nebulae. 



3 



small stars, which will without doubt be found of the greatest importance 

 in the future study of their proper motions. D'Arrest's work "Sid. Neb. 

 Observationes Havnienses" in particular is extremely rich in determinations 

 of this kind. 



The first series of determinations of the positions of nebulae, made on 

 the other hand exclusively for the purpose of forming a basis to the study 

 of their proper motion, dates trom a period no longer back than about 20 

 years, and consists, as has been already intimated, of Laugier s observations 

 of 53 brighter nebula} made at Paris and D'Arrest's Leipzig-observations of 

 more than 200 nebulae. These two series of observations, which were both 

 executed with small instruments, appear to be of about equal accuracy and 

 the probable error in the definitive positions to amount to about 6". These 

 works have the high merit of having originated a new and important branch 

 in the study of the nebulae; and D'Arrest has done especial service to this 

 study, by showing, that when what is required is simply good determina- 

 tions of positions, a much greater number of nebula 1 than has been usually 

 supposed may be advantageously observed with instruments of but very 

 moderate dimensions. But his series of observations is chiefly and especi- 

 ally important as proving beyond the possibility of a doubt, that the po- 

 sitions of nebulae in general are determinable with far greater accuracy than 

 it had been previously usual to suppose; and D'Arrest's work thus made 

 an epoch in the study of nebulae by freeing it from deterring prestige which 

 had before that period been attached to it. D'Arrest soon found followers, 

 who prosecuted his so happily commenced work. We have accordingly 

 latterly been provided with accurate positions of nebulae by Auvers, Schmidt, 

 Schonfeld, Vogel, Rumker, Stephan and others; there are moreover ad- 

 mirable monographies in this way, as for example Bond's and Liapounov's 

 excellent measures of the stars in the great nebula of Orion. Among the 

 general series of observations of this kind Schonfeld's famous Maunheim- 

 observations of 235 nebulae are the most comprehensive; and, as the pro- 

 bable error in one of Schonfeld's positions appears not to exceed 2" or 3", 

 and in for exemple those of Auvers and Vogel to be still less, it is evi- 

 dent that by a sufficient multiplication of observations the determination of 

 positions may be easily carried to a very high degree of accuracy, as indeed 

 my own experience has shown me. 



My own observations, which were commenced in 1863, and have ever 

 since been continued with more or less regularity, consist of determinations 

 by means of a wire-micrometer of differences of right ascension and decli- 

 nation between about 500 nebulae and neighbouring stars of sufficient bright- 



