The unaided eye, with an available area of one-twentieth of a square inch, permits us to see stars of the sixth magnitude. Herschel's 18-inch reflector, with an area 5,000 times as great, rendered visible stars of the fifteenth magnitude. The 60-inch reflector, with an area 57,600 times that of the eye, reveals stars of the eighteenth magnitude, while to reach stars of about the twentieth magnitude, photographic exposures of four or five hours suffice with this instrument. Every gain of a magnitude means a great gain in the number of stars rendered visible. Stars of the second magnitude are 3.4 times as numerous as those of the first, those of the eighth magnitude are three times as numerous as those of the seventh, while the sixteenth magnitude stars are only 1.7 as numerous as those of the fifteenth magnitude. This steadily decreasing ratio is probably due to an actual thinning out of the stars toward the boundaries of the stellar universe, as the most exhaustive tests have failed to give any evidence of absorption of light in its passage through space. But in spite of this decrease, the gain of a single additional magnitude may mean the addition of many millions of stars to the total of those already shown by the 60-inch reflector. Here is one of the chief sources of interest in the possibilities of a 100-inch reflecting telescope. 100-INCH TELESCOPE [Illustration: Fig. 10. One-hundred-inch mirror, just silvered, rising out of the silvering-room in pier before attachment to lower end of telescope tube. (Seen above.)] In 1906 the late John D. Hooker, of Los Angeles, gave the Carnegie Institution of Washington a sum sufficient to construct a telescope mirror 100 inches in diameter, and thus large enough to collect 160,000 times the light received by the eye. (Fig. 10.) The casting and annealing of a suitable glass disk, 101 inches in diameter and 13 inches thick, weighing four and one-half tons, was a most difficult operation, finally accomplished by a great French glass company at their factory in the Forest of St. Gobain. A special optical laboratory was erected at the Pasadena headquarters of the Mount Wilson Observatory, and here the long task of grinding, figuring, and testing the mirror was successfully carried out by the observatory opticians. This operation, which is one of great delicacy, required years for its completion. Meanwhile the building, dome, and mounting for the telescope were designed by members of the observatory staff, and the working drawings were prepared. An opportune addition by Mr. Carnegie to the endowment of the Carnegie Institution of Washington, of which the observatory is a branch, permitted the necessary appropriations to be made for the completion and erection of the telescope. Though delayed by the war, during which the mechanical and optical facilities of the observatory shops were utilized for military and naval purposes, the telescope is now in regular use on Mount Wilson.