362 DISTANCE OF THE STARS. SECT. XXXVII. 



nomena. The whole number of stars registered amounts 

 to about 150,000 or 200,000. The distance of the fixed 

 stars is too great to admit of their exhibiting a sensible 

 disc ; but in all probability they are spherical, and must 

 certainly be so if gravitation pervades all space, which it 

 may be presumed to do, since Sir John Herschel has 

 shown that it extends to the binary systems of stars. 

 With a fine telescope the stars appear like a point of 

 light ; their occultations by the moon are therefore 

 instantaneous. Their twinkling arises from sudden 

 changes in the refractive powers of the air, which would 

 not be sensible if they had discs like the planets. Thus 

 we can learn nothing of the relative distances of the 

 stars from us, and from one another, by their apparent 

 diameters. The annual parallax of all but a very few 

 being insensible, shows we must be more than two 

 hundred millions of millions of miles at least from them. 

 Many of them, however, must be vastly more remote ; 

 for of two stars that appear close together, one may be 

 far beyond the other in the depth of space. 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. 



Nothing is known of the absolute magnitude of the 

 fixed stars, but 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 them- 

 selves. By this method he found that the light of the 

 sun is about twenty millions of millions of times greater 

 than that of Sirius, the brightest and one of the nearest 

 of the fixed stars. Since the parallax of Sirius is about 

 half a second, its distance from the earth must be 592,200 

 tim es the distance of the sun from the earth ; and 

 therefore Sirius, placed where the sun is, would appear 



