Brighter than "zero magnitude" Sirius (-1·58); Canopus (-0·86) Zero magnitude α Centauri (0·06) 0 to 0·4 magnitude Vega (0·14); Capella (0·21); Arcturus (0·24); Rigel (0·34) 0·5 magnitude Procyon (0·48) 1st " Aldebaran (1·06) 2nd " α Persei (1·90); β Aurigæ (2·07) 3rd " η Boötis (3·08); ζ Capricorni (2·98) 4th " ρ Leonis (3·85); λ Scorpii (4·16); γ Crateris(4·14); ρ Herculis (4·14) 5th " ο Pegasi (4·85); μ Capricorni (5·10) CHAPTER XX The Visible Universe Some researches on the distribution of stars in the sky have recently been made at the Harvard Observatory (U.S.A.). The principal results are:--(1) The number of stars on any "given area of the Milky Way is about twice as great as in an equal area of any other region." (2) This ratio does not increase for faint stars down to the 12th magnitude. (3) "The Milky Way covers about one-third of the sky and contains about half of the stars." (4) There are about 10,000 stars of magnitude 6·6 or brighter, 100,000 down to magnitude 8·7, one million to magnitude 11, and two millions to magnitude 11·9. It is estimated that there are about 18 millions of stars down to the 15th magnitude visible in a telescope of 15 inches aperture.[456] According to Prof. Kapteyn's researches on stellar distribution, he finds that going out from the earth into space, the "star density"--that is, the number of stars per unit volume of space--is fairly constant until we reach a distance of about 200 "light years." From this point the density gradually diminishes out to a distance of 2500 "light years," at which distance it is reduced to about one-fifth of the density in the sun's vicinity.[457] In a letter to the late Mr. Proctor (_Knowledge_, November, 1885, p. 21), Sir John Herschel suggested that our Galaxy (or stellar system) "contained within itself miniatures of itself." This beautiful idea is probably true. In his account of the greater "Magellanic cloud," Sir John Herschel describes one of the numerous objects it contains as follows:--