495 



The author observes, however, that in the instances which present 

 themselves of such connexion between stars and nebulae, which are 

 the two extremes of the series, the nebulosity may not always be a 

 remnant of the unsubsided nebulous matter from which they were 

 originally formed, but detached portions of nebulous matter may, 

 like stars, have a considerable proper motion, and may be intercepted 

 in their course by clusters of stars, or by the more powerful attrac- 

 tion of a single star of great magnitude, by which they will in still 

 less time be absorbed. In Dr. Herschel's endeavours to arrange the 

 vast accumulation of observations already recorded on this subject, 

 there are many phenomena too ambiguous to admit of classification ; 

 but this, he observes, will necessarily occur at every period in the 

 progressive improvement of telescopes ; since a greater power of pe- 

 netrating into space, which would be sufficient to render all present 

 objects distinct, would bring into view a still greater number of ap- 

 pearances, requiring a still further extension of our powers to com- 

 prehend. 



After arranging the various instances of gradation in which nebu- 

 losity appears successively more and more condensed, whether with 

 or without intervening stars, the author examines aggregations of 

 stars alone, referring to many former observations of patches of stars, 

 which, at the time of recording them, he was induced to call forming 

 clusters, in consequence of some appearances of a tendency to approach, 

 which he inferred from the greater density of such clusters toward 

 their centre. This apparent propensity to cluster seemed chiefly 

 visible in parts of the heavens extremely rich in stars ; and Dr. Her- 

 schel refers to about 150 instances of such an appearance in the Milky 

 Way, but generally of an irregular form', and very imperfectly col- 

 lected. Of other clusters, in which more of regularity is observable, 

 a more particular description is given. 



The various degrees of compression of different clusters are also 

 noticed, with references to numerous instances by classes in which 

 they are now arranged. Some of these are visible with ordinary 

 telescopes ; others are selected as fine objects for good telescopes ; 

 and others again, on account of their higher compression, cannot be 

 resolved without the aid of very superior telescopes. 



The form, also, of those most compressed is observed to partake 

 more or less of a spherical form. Thirty-nine instances are quoted in 

 which the form is oval in various degrees. But objects of this de- 

 scription can hardly be seen to advantage without a twenty-feet 

 telescope. Others again, and very numerous, are referred to, disco- 

 vered as globular nebulae with common telescopes, but resolved into 

 stars by telescopes of high magnifying as well as space-penetrating 

 power ; and as these are accordingly but little known, Dr. Herschel 

 selects, from numerous observations that he has made during four- 

 and-thirty years, various nebulae, classed according to the telescopes 

 with which he had observed them, as a guide to those who may wish 

 to view them, that they may be able to judge which objects may pos- 

 sibly be within the power of the telescopes they happen to possess. 



