ck Observatory; the great 72-inch reflector,
recently brought into effective service at the Dominion Observatory
in Canada; and the 60-inch and 100-inch reflectors of the Mount
Wilson Observatory.

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 car