The number of the stars is not less amazing than their distance. It is true that the number visible to the unaided eye is not by any means so great as might be imagined on a casual survey. On a clear night the eye receives the impression that the multitude of stars is so great as to be utterly beyond counting; but this is not the case. The naked-eye, or 'lucid,' stars have frequently been counted, and it has been found that the number visible to a good average eye in both hemispheres together is about 6,000. This would give for each hemisphere 3,000, and making allowance for those lost to sight in the denser air near the horizon, or invisible by reason of restricted horizon, it is probable that the number of stars visible at any one time to any single observer in either hemisphere does not exceed 2,500. In fact Pickering estimates the total number visible, down to and including the sixth magnitude, to be only 2,509 for the Northern Hemisphere, and on that basis it may safely be assumed that 2,000 would be the extreme limit for the average eye. [Illustration: PLATE XXVII. Region of the Milky Way in Sagittarius. Photographed by Professor E. E. Barnard.] But this somewhat disappointing result is more than atoned for when the telescope is called in and the true richness of the heavenly host begins to appear. Let us take for illustration a familiar group of stars--the Pleiades. The number of stars visible to an ordinary eye in this little group is six; keen-sighted people see eleven, or even fourteen. A small telescope converts the Pleiades into a brilliant array of luminous points to be counted not by units but by scores, while the plates taken with a modern photographic telescope of 13 inches aperture show 2,326 stars. The Pleiades, of course, are a somewhat notable group; but those who have seen any of the beautiful photographs of the heavens, now so common, will know that in many parts of the sky even this great increase in number is considerably exceeded; and that for every star the eye sees in such regions a moderate telescope will show 1,000, and a great instrument perhaps 10,000. It is extremely probable that the number of stars visible with the largest telescopes at present in use would not be overstated at 100,000,000 (Plate XXVII.).