125 respectively, the illuminating powers of the two telescopes are
inferred to be as 678 to 290.
The mean of this and the two former experiments, without making allowance for the imperfect polish of the Cassegrainian in one of them, gives the comparative superiority in the illuminating power of the Cassegrainian more than 2 to 1 ; orif the experiment which the au- thor considers less perfect be rejected, it would appear to be 2-} to l in favour of the Cassegrainian construction.
In a former communication to the Society, the author endeavoured to show the probability of a progressive condensation of nebulous matter, so as to put on ultimately the appearance of stars : his pre- sent object is to show, by reference to select cases from many thou- sand former observations on record, that a similar operation of gra- dual condensation is also taking place among condensed clusters of visible stars, and consequently to render it probable that an intimate connexion subsists between these extremes, and that the same pro- cess of condensation continues from one end of the series to the other; so that the most perfect stars may possibly have originated from an accumulation of mere nebulous matter.
His first observations, indeed, relate to a more direct communicaa tion between present stars and contiguous nebulosity in different relative positions. In some instances a single star appears to be attracting to itself a branch of nebulosity, seen extending from one of its sides; in others, two adjacent stars appear to have equal power over a linear portion of nebula that extends from one to the other.
The portions of nebula, however, that are adjacent to different single stars, vary considerably in their appearance. Of those nebulous branches that extend only on one side, some have parallel sides, some are fan-shaped, others are in a considerable degree irregular. Some stars have extended nebulosity on opposite sides, in a line of which they occupy the centre. Round others it appears diffused equally, as in a globular form, on all sides; and in some instances, such a globular nebula apparently includes a cluster of several stars together. All these appearances afford a presumption, that stars and nebula are drawn together by mutual attraction, and that the accession of such a quantity of matter as must be contained in an extensive nebula will ultimately cause what may be called the growth of stars. What in its first state appeared as a globular nebula alone, would, by condensation, present the appearance of a nucleus in its centre. The globular nebula with nucleus would, by increasing condensation, become a nebulous star; its next state would be that of a distinct star, with surrounding nebulosity ; and the last result would be the perfect simple star.
