404 COMPARATIVE LIGHT OF STABS. SECT. XXXVI. 



the sun in a second will give the ratio of the mass of the great 

 star to that of the sun or earth. According to M. Bessel, the 

 weight of the two stars of Gl Cygni is equal to half the weight of 

 the sun. Little as we know of the absolute magnitude of the 

 fixed stars, the quantity of light emitted by many of them shows 

 that they must be much larger than the sun. Dr. Wollaston 

 determined the approximate ratio which the light of a wax candle 

 bears to that of the sun, moon, and stars, by comparing their 

 respective images reflected from small glass globes filled with 

 mercury, whence a comparison was established between the 

 quantities of light emitted by the celestial bodies themselves. By 

 this method he found that the light of Lyras is five and a half 

 times greater than that of the sun. Sir John Herschel reflected 

 the moon's light totally by a prism, which, concentrated by a 

 lens, was compared directly with that of Centauri. After 

 making allowance for the quantity of the moon's light lost in 

 passing through the lens and prism, he found that the mean 

 quantity of light sent to the earth by a full moon exceeds that 

 sent by Centauri in the proportion of 27,408 to 1. Now, 

 Dr. Wollaston found the proportion of the sun's light to that of 

 the full moon to be that of 801,072 to 1. Hence, the light sent 

 to us by the sun is to that sent by Centauri as about twenty- 

 two thousand millions to one. But, as the parallax of Centauri 

 is 1", it really is two and a half times brighter than the sun. 

 The light of Sirius is four times that of a Centauri, but its 

 parallax is only 0"'230: hence it has an intrinsic splendour 63'02 

 times that of our luminary. It is therefore estimated to be a 

 hundred times as large ; so that, were Sirius in the earth's place, 

 its surface would extend 150 times as far as the orbit of the 

 moon. The light of Sirius, according to the observations of 

 Sir John Herschel, is 324 times greater than that of a star of the 

 sixth magnitude ; if we suppose the two to be really of the same 

 size, their distances from us must be in the ratio of 57 '3 to 1, 

 because light diminishes as the square of the distance of the 

 luminous body increases. 



So many of the stars have proper motions altogether indepen- 

 dent of the annual rotation of the earth in its orbit, that it may 

 be doubted whether there be such a thing as a fixed star. 

 Groombridge is the most rapid known : it has a proper motion of 

 7" of arc annually ; Centauri moves at the rate of 3"*58 annu- 



