other clusters in Perseus represented on the map are worth examining,
although none of them calls for special mention, except perhaps 584,
where we may distinguish at least a hundred separate stars within an
area less than one quarter as expansive as the face of the moon.

Among the double stars of Perseus we note first eta, whose components
are of magnitudes four and eight, distance 28", colors white and pale
blue. The double epsilon is especially interesting on account of an
alleged change of color from blue to red which the smaller star
undergoes coincidently with a variation of brightness. The magnitudes
are three and eight, distance 9", p. 9 deg.. An interesting multiple is
zeta, two of whose stars at least we can see. The magnitudes are three,
nine, ten, and ten, distances 13", p. 207 deg., 90", and 112".

The chief attraction in Perseus is the changeful and wonderful beta, or
Algol, the great typical star among the short-period variables. During
the greater part of its period this star is of magnitude two and two
tenths, but for a very short time, following a rapid loss of light, it
remains at magnitude three and seven tenths. The difference, one
magnitude and a half, corresponds to an actual difference in brightness
in the ratio of 3.75 to 1. The entire loss of light during the
declension occupies only four hours and a half. The star remains at its
faintest for a few minutes only before a perceptible gain of light
occurs, and the return to maximum is as rapid as was the preceding
decline. The period from one minimum to the next is two days twenty
hours forty-eight minutes fifty-three seconds, with an irregularity
amounting to a few seconds in a year. The Arabs named the star Algol, or
the Demon, on account of its eccentricity which did not escape their
attention; and when Goodricke, in 1782, applied a scientific method of
observation to it, the real cause of its variations was suggested by
him, but his explanation failed of general acceptance until its truth
was established by Prof. E. C. Pickering in 1880. This explanation gives
us a wonderful insight into stellar constitution. According to it, Algol
possesses a companion as large as the sun, but invisible, both because
of its proximity to that star and because it yields no light, and
revolving in a plane horizontal to our line of sight. The period of
revolution is identical with the period of Algol's cycle of variation,
and the diminution of light is caused by