76 Recent Advances in Stellar Astronomy
luminous rings, separated by dark spaces and each fainter
than the last. Even if the object at which the telescope
was directed were a mathematical point, its image would
still show this spurious disk, and, for a larger object, the
image of each point is similarly expanded, so that the
whole effect is blurred. No perfection of workmanship
can eliminate this difficulty, which is inherent in the very
nature of light; but it may be reduced by using a telescope
of large size, for, the larger the aperture of the telescope,
the smaller is the spurious image. For a one-inch aperture,
it is 4".5 in diameter, but for a ten-inch it is only one-
tenth as large. For the 40-inch Yerkes telescope, the dif-
fraction disk measures only 0".ll, and for the 100-inch
at Mount Wilson it should be slightly less than 0".05. A
large telescope, therefore, is inherently superior to a small
one in its capacity for revealing fine detail, such as close
double stars. When, with a small instrument, the spurious
disks would overlap and conceal all trace of duplicity, a
great one shows the components sharply separated—pro-
vided, at least, that the air is steady. On nights when the
air is not optically homogeneous—which, alas, are in the
large majority at most places—there is a continuous “boil-
ing” or disturbance of the image, which seriously increases
the blurring, and too often renders all refined observations
impossible.
From the known distances of the stars, and the real
diameters already estimated, it is easy to show that, even
in the most favorable instances, the angular diameter of a
star will not exceed 0".05, and that cases in which it is
greater than 0".01 are rare. The real disks of the stars,
therefore, must be smaller than the spurious disks pro-
duced by diffraction in even the greatest existing telescopes,
and all that we can hope to see, even under ideal atmos-