Astronomical Society. 227 



ASTRONOMICAL SOCIETY OF LONDON. 



Jan. 11, 1828. — There was read a paper entitled "Third Series of 

 Observations with a 20-feet reflecting telescope ; — containing a Ca- 

 talogue of 384 new double and multiple stars, completing a first 

 thousand of those objects detected in sweeps with that instrument j — 

 together with Observations of some previously known." By J. F. W. 

 Herschel, Esq., President of the Society. 



This paper, as its title imports, is a continuation of the two papers 

 previously communicated by the author on the same subject. The 

 field of discovery in this department of astronomy, though narrowed 

 by the great work recently published by Professor Struve, the author 

 considers as not yet exhausted; since, on an average of the part of 

 the heavens swept by him, not above one in four, of double stars 

 sufficiently remarkable to attract attention in sweeping, have been 

 catalogued by the eminent astronomer last named : not to mention 

 the vast number of interesting close double stars, below the 9th 

 magnitude, which a minuter examination than the nature of his 

 sweeps permits would no doubt produce. The double stars of this 

 Catalogue, he observes, are considerably more select than those of his 

 two former ones; those whose distance exceeds 32" being (except in 

 particular cases) excluded, and the limit of distance being narrowed 

 according to the faintness of the component stars. 



The author prefaces his Catalogue with a comparison of the mag- 

 nitudes habitually assigned to the stars by himself and Professor' 

 Struve j from which it appears that on the average, his magnitudes 

 have a denomination about one unit lower than those of that astro- 

 nomer ; — a star (for example) which M. Struve would call of the 9th 

 magnitude, being, in Mr. Herschel's nomenclature, of the 10th. The 

 limit of vision in the Dorpat telescope, he presumes to lie about his 

 average 14th magnitude, though such a determination must neces- 

 sarily be liable to some latitude. This conclusion he deduces from 

 a series of instances, in which small companions have been seen by 

 him attached to large stars, within the limits of Professor Struve's 

 4th class, which have escaped the notice of the latter. 



The author then states the principle on which he estimates magni- 

 tudes below the 6th, which is that of continual bisection of the light; 

 and he cites some experiments, by which it appears that the light of 

 an average star of the 1st magnitude is at least lf)0 times that of the 

 6th. He then adduces a series of observations of a considerable 

 number of the closer stars, of M. Struve's Catalogue, by which it 

 appears that the Slough telescope easily defines with its ordinary 

 sweeping power, the generality of M. Struve's stars of the 1st class, 

 and many of those marked by him as vicince, and even pervicince ; but 

 those which have the epithet vicinissimce, he has not yet succeeded 

 in separating with the highest power (240) usually applied, — which 

 indeed was to be expected. In lieu of M. Struve's classification of 

 double stars, which he considers as enlarging beyond due limits the 

 number of those of the 1st class, he proposes the following system, 

 which in fact very nearly approximates to that originally followed by 

 Sir William Herschel. 



2 G 2 Class 



