Noflces respecting New Booh. 625 



superior in lustre to Arcturus. All observers are agreed in regarding- 

 the principal star as of the first magnitude. Lacaille and others 

 down to Sir John Herscliel have estimated the smaller star as of 

 the fourth. Sir John says his habitual judgement has inclined him 

 rather to the first tb.an the second magnitude, and he is disposed to 

 assign to it a magnitude which ma}^ be deemed indifferently either 

 a very low first or a very high second. And he accounts for the very 

 different estimates given of its magnitude by former observers to the 

 known fact (whatever the cause may be) that the apparent inequality 

 of two stars seen at once in the same field of view diminishes as the 

 light of the telescope is greater. 



Among the circumstances which render this star remarkable, one 

 is its very great proper motion, which, according to the determination 

 of the late Professor Henderson, amounts to 3"'.58 in a j^ear, in the 

 arc of a great circle. This circumstance affords in the first place a 

 very strong presumption that the two stars are physically connected ; 

 for the chances may l)e regarded as almost infinite against the agree- 

 ment of the proper motions of two independent stars, both in quantity 

 and direction ; and that such agreement exists in the present case in 

 both respects is manifest, for if the motion of translation through 

 space were not common to the two stars, the one would, in the cen- 

 tury which has elapsed since the observations of Lacaille, have left 

 the other behind nearly 5' in arc. 



But the existence of a large proper motion affords also s. primd facie 

 reason for presuming the relatively greater proximity of the star to 

 the earth, by which the apparent motion, whether referred to the star 

 or to the translation of the solar system, would be increased. The 

 j)resumption of proximity thus arising has been confirmed in the 

 present instance by Professor Plenderson, who deduced from his 

 meridional observations made at the Cape an annual parallax amount- 

 ing to about T' of arc. It is this circumstance which renders a Cen- 

 tauri an object of so much interest to astronomers, for with the 

 exception of 61 Cygni (also double), there is no star whose distance 

 can be regarded as determined with the same certaintjr ; and in the 

 case of the latter, the parallax, as found by Bessel, scarcely exceeds 

 one-third of a second. 



The micrometrical measures of a Centauri give no decided evi- 

 dence of angular motion. Sir John however observes, that there can 

 be no doubt that the distance between the two stars has gone on 

 steadily increasing since i8"i-2 at least. The small amount of varia- 

 tion in the angle of position shows that the plane of the orbital mo- 

 tion (supposing such to exist) passes nearly, but not quite, through 

 our system, while its actual tendency to increase exemplifies the 

 general law of increase of angular velocity with diminution of 

 distance. He finds also, from a comjiarison of measures, that 

 the major axis of the orbit must materially exceed 24" ; whence, 

 taking the coefficient of jjarallax at the amount given by Henderson, 

 it will follow that the real diameter of the relative orbit of the com- 

 panion-star cannot be so small as that of the orbit of Saturn, and 

 exceeds in all probability that of the orbit of Uranus. 



