The author obscures, 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 maybe 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 suflicient to render all present
objects distinct. would 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 consequende of some appearances of a tendency to approach. which be 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 numerons 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 nebula, 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.
