III. THE STELLAR UNIVERSE. I. GENERAL ASPECT OF THE HEAVENS. 322. _The Magnitude of the Stars._--The stars that are visible to the naked eye are divided into six classes, according to their brightness. The brightest stars are called stars of the _first magnitude_; the next brightest, those of the _second magnitude_; and so on to the _sixth magnitude_. The last magnitude includes the faintest stars that are visible to the naked eye on the most favorable night. Stars which are fainter than those of the sixth magnitude can be seen only with the telescope, and are called _telescopic stars_. Telescopic stars are also divided into magnitudes; the division extending to the _sixteenth_ magnitude, or the faintest stars that can be seen with the most powerful telescopes. The classification of stars according to magnitudes has reference only to their brightness, and not at all to their actual size. A sixth magnitude star may actually be larger than a first magnitude star; its want of brilliancy being due to its greater distance, or to its inferior luminosity, or to both of these causes. None of the stars present any sensible disk, even in the most powerful telescope: they all appear as mere points of light. The larger the telescope, the greater is its power of revealing faint stars; not because it makes these stars appear larger, but because of its greater light-gathering power. This power increases with the size of the object-glass of the telescope, which plays the part of a gigantic pupil of the eye. The classification of the stars into magnitudes is not made in accordance with any very accurate estimate of their brightness. The stars which are classed together in the same magnitude are far from being equally bright. The stars of each lower magnitude are about two-fifths as bright as those of the magnitude above. The ratio of diminution is about a third from the higher magnitude down to the fifth. Were the ratio two-fifths exact, it would take about 2-1/2 stars of the 2d magnitude to make one of the 1st. 6 stars of the 3d magnitude to make one of the 1st. 16 stars of the 4th magnitude to make one of the 1st. 40 stars of the 5th magnitude to make one of the 1st. 100 stars of the 6th magnitude to make one of the 1st. 10,000 stars of the 11th magnitude to make one of the 1st. 1,000,000 stars of the 16th magnitude to make one of the 1st.