IV INTRODUCTION. 



tieularly as difficulties frequently arise as to the date of a paper, 

 which it is often hard to fix except by some arbitrary rule. If 

 such a rule be once adopted it must be adhered to although 

 it will often disarrange the true logical order of the papers. 

 Third, the question may be to discover all the papers written 

 on a special subject, no matter by what author. In the present 

 case such a question might be to find the papers which have 

 been written on a given nebula, or on the spectra of nebula? in 

 general, or something allied to this. It is plain that in so new 

 a subject great subdivision is not required, and I find by experi- 

 ence that all my own wants in this direction have been supplied by 

 adding after the reference to each paper a note more or less ex- 

 tended, giving an indication of the purport of the paper. A slight 

 familiarity with the writings of the various authorities will enable 

 one readily to turn to the authors likely to have treated a given 

 subject. Thus if the proper motion of nebuhc was in question, the 

 writings of Otto Struve, D' Arrest, Schoenfeld, Vogel, 

 Schultz, Schmidt, Rumker, Argelander, Oppolzer, Win- 

 NECKE,etc., would naturally be first consulted, and so with other 

 subjects. 



One nebula, that in Orion, has received a vast amount of study, 

 and I have added a special bibliography of this, as well as one 

 referring to the Variable Nebulse. 



Section IV consists of a reference-list to all published (and 

 many unpublished) drawings of nebulae and clusters. Under this 

 head many of the most extended memoirs will be again indexed, 

 and nearly all the monographs, so that it supplements in a way the 

 main index. That is if the memoirs on a certain nebula are sought 

 for and no preliminary clue is at hand, (as to authors,) it will be 

 Avell before looking through the whole list of author's names, to 

 look for this nebula in the list of drawings. If it is found there, 

 the references joined to it will indicate the principal papers on this 

 object. 



To facilitate the finding of nebulse I have given in this part oi 



