42 Recent Advances in Stellar Astronomy
all, and yet escape being blinded by the glare of day. But
our eyes do better than merely to detect the almost in-
finitesimal quantities of energy which the stars send us:
they give us very good reports regarding their relative
amounts.
Almost from time immemorial, the stars have been
classified according to their apparent brightness, and the
system of describing this which is still used is well over
two thousand years old. Since the days of Hipparchus
the brightest of them have been called “stars of the first
magnitude,” those decidedly fainter, “of the second magni-
tude,” and so on to the sixth magnitude—the faintest
visible to the unaided eye. When accurate measures of
starlight began to be made, about the middle of the nine-
teenth century, it was found that a 2nd magnitude star sent
us about 2% times as much light as one of the 3rd magni-
tude, the latter again 2j4 times the light of a 4th magnitude
star, and so on—a difference of five magnitudes corre-
sponding to a light ratio of 100. With this principle as
guide, the scale of magnitudes can be extended to the
faintest stars visible with the telescope, which in the case
of the hundred-inch instrument at Mount Wilson are of
about the 21st magnitude, and backward to bright objects,
for which the numbers expressing the magnitudes are nega-
tive, so that the magnitude of Venus is about —4; of the
full Moon —12.5 ; and of the Sun —26.7.
We are not confined, however, to visual observations,
when we measure the brightness of the stars. The
intensity of the image on a photograph, if properly
standardized, serves as well, and the photographic magni-
tudes which are thus derived give us much more than a
mere check upon our previous results. The eye is sensi-
tive primarily to green and yellow light (red and blue, for