494 



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 ; or if the experiment which the au- 

 thor considers less perfect be rejected, it would appear to be 2-J- to 1 

 in favour of the Cassegrainian construction. 



Astronomical Observations relating to the sidereal part of the Heavens, 

 and its Connexion with, the nebulous part ; arranged for the purpose 

 of a critical Examination. By William Herschel, LL.D. F.R.S. 

 Read February 24, 1814. [Phil Trans. 1814, p. 248.] 



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 communica- 

 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 nebu- 

 lous 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 

 nebulae are drawn together by mutual attraction, and that the acces- 

 sion of such a quantity of matter as must be contained in an exten- 

 sive 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 con- 

 densation, 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. 



