Page:The fairy tales of science.djvu/225

Rh, is that into classes, according to their apparent brightness. These classes astronomers term magnitudes. The brightest stars are said to be of the first magnitude; those next in brightness of the second magnitude, and so forth. The stars down to the sixth magnitude are visible to the naked eye; it requires, however, tolerably good eyes to distinguish those of the sixth magnitude, even on very clear evenings. For stars below the sixth magnitude we must have recourse to telescopes; with the aid of the most powerful of these instruments, we can at present discern stars down to the twentieth magnitude, and even below. The number of stars of the first magnitude is very small, only about 20 of them being counted in the heavens; those of the second magnitude number 65; of the third, 190; of the fourth, 425; of the fifth, 1100; of the sixth, 3200; of the seventh, 13,000; of the eighth, 40,000; of the ninth, 142,000,—which gives a total number of 200,000 stars down to the ninth magnitude. As a glance at these figures will show, the numbers increase very rapidly as we descend in the scale of brightness. To conceive a notion, still most inadequate, however, of the countless multitudes of stars that are dispersed through infinite space, we need simply reflect that Sir William Herschel, through his powerful telescope, discovered some eighteen millions of stars, of an average magnitude between the tenth and eleventh, in the milky way alone—that great luminous band which stretches all across the