only twenty stars of the first magnitude, that is, of the first degree of brightness. It is demonstrable that Sirius is a hundred and sixty times as bright intrinsically as our sun. From a comparison of Sirius with the moon, and the moon with the sun, it is found that sunlight is 6,000 million times brighter than that of Sirius. But since Sirius is 1,000,000 times more distant than our sun, if brought as near us, the intensity of its light would be increased in the ratio of the square of 1,000,000 or 1,000,000 millions. This quantity, divided by 6,000 millions, would show Sirius to be at least a hundred and sixty times as bright intrinsically as our sun.
It is fair, however, to suppose that the chief cause of difference of brightness of stars is difference of distance. If so, we then have the means of approximating to the distance of even telescopic stars. Herschel estimated that stars of the first order emit, on an average, light one hundred times as intense as those of the sixth; hence the latter must be at ten times the distance, the intensity of light varying inversely as the square of the distance. Sirius is 98,000,000 of millions of miles from us; stars of the sixth magnitude are therefore about 1,000 millions of millions distant, and their light reaches us only after a flight of 169 or 170 years.
The space-penetrating power of a telescope depends upon the ratio between its aperture and the pupil of the eye. Herschel estimated that his four-foot reflector penetrated 194 times as far as the naked eye; and, as the faintest stars visible to the naked eye are ten times as distant as the brightest, it follows that the faintest stars visible in the field of Herschel's telescope are 1,940 times as distant as Sirius or Alpha Lyræ. In other words, Sirius would still be visible if removed to about 2,000 times its present distance. With Lord Rosse's six-foot reflector Sirius would still be visible at 3,000 times its present distance. If, therefore, this brightest star in the heavens should suddenly shoot away from us with the velocity of light, it would remain visible to the naked eye twice 170 years, since 170 years would be consumed in its flight and 170 more in the return of the light; and with Lord Rosse's telescope it could be traced 100,000 years longer (twice 3,000 times 170 years). This is the lowest estimate, too, for Sirius is many times brighter than the average star of the first magnitude. If intrinsically only four times as bright, it could still be seen by the telescope so infinitely far into space that light from it could reach us only after the lapse of 100,000 years.
But we need not pause even at these distant telescopic suns. Beyond the stars that merely dot with points of light the telescopic field of view, hovering on the outskirts of this inconceivable space swept by the far-fathoming line of telescopic vision, are discerned faintly-defined masses too distant to be resolved into stars, whose hazy light is gathered from congeries of suns, the individual blazing brightness of which is reduced in our view to the fleecy films of unresolvable nebulæ. Even