INTRODUCTION. 



The earliest systematical study of the nebulae is contemporaneous with 

 the rise of stellar astronomy under the auspicies of William Hekschel, and 

 may half a century be said to have been a kind of Herschelian science, 

 with which scarcely any others than the Herschel family occupied themsel- 

 ves. It is only within the last few decennia that some few other men of 

 science, e. g. Lord Rosse, Lassel, D'Arkest and some others have begun 

 to turn their attention in this direction and with more or less magnificient 

 instruments have prosecuted the researches in this quarter commenced by 

 Sir William and Sir John Herschel. 



The first question as regards these objects, that observation had to 

 decide, was that of their absolute and relative frequency in the heavens, 

 and the first problem was accordingly to determine on a large scale with 

 sufficient accuracy for the purpose approximate positions, so as to obtain a 

 catalogue of nebulae as comprehensive as possible. But as these objects 

 differ widely in their external appearance, and thus offerd the greatest va- 

 riety in singular forms, size, brilliancy, etc., it further became a matter of 

 great importance, by means of accurate descriptions and figures of the 

 occurring nebula- types , to endeavour to gain some knowledge of their 

 apparent nature in general, so as to provide material for the possible study 

 of their physical construction. In this respect the resolubility for example 

 of the nebulae and their variability as regards form and brilliancy became 

 questions of the first importance. Hence soon followed the classification of 

 the nebulae in more or less natural groups, the chief value of which lay 

 in the facilities they offered for prosecuting the study of the objects' peculiar 

 properties. The fundamental works on this subject, not only those of a 

 general character but sonographies referring to particular remarkable ne- 

 bulae, accordingly treat almost exclusively the above mentioned chief points 

 of nebular study, even when we include the spectro-analytical researches of 

 late years instituted by Huggins, D' Arrest and others. 



Nova Acta Reg. Soc. Sc. Ups. Ser. HI. 1 



