the interposition of the dark
body as it sweeps along that part of its orbit lying between our point
of view and the disk of Algol. In other words, once in every two days
twenty hours and forty-nine minutes Algol, as seen from the earth,
undergoes a partial eclipse.

In consequence of the great comparative mass of its dark companion,
Algol itself moves in an orbit around their common center with a
velocity quite sufficient to be detected by the shifting of the lines in
its spectrum. By means of data thus obtained the mass, size, and
distance apart of Algol and its singular comrade have been inferred. The
diameter of Algol is believed to be about 1,125,000 miles, that of the
dark body about 840,000 miles, and the mean distance from center to
center 3,230,000 miles. The density of both the light and the dark star
is slight compared with that of the sun, so that their combined mass is
only two thirds as great as the sun's.

Mention has been made of a slight irregularity in Algol's period of
variation. Basing his calculations upon this inequality, Dr. Chandler
has put forward the hypothesis that there is another invisible body
connected with Algol, and situated at a distance from it of about
1,800,000,000 miles, and that around this body, which is far more
massive than the others, Algol and its companions revolve in a period of
one hundred and thirty years! Dr. Chandler has earned the right to have
his hypotheses regarded with respect, even when they are as
extraordinary as that which has just been described. It needs no
indulgence of the imagination to lend interest to Algol; the simple
facts are sufficient. How did that bright star fall in with its black
neighbors? Or were they created together?

[Illustration: MAP NO. 25.]

Passing to the region covered by map No. 25, our eyes are caught by the
curious figure, formed by the five brightest stars of the constellation
Cassiopeia, somewhat resembling the letter W. Like Perseus, this is a
rich constellation, both in star clusters and double stars. Among the
latter we select as our first example sigma, in which we find a
combination of color that is at once very unusual and very
striking--green and blue. The magnitudes are five and seven, distance
3", p. 324 deg.. Another beautiful colored double is eta, whose magnitudes
are four and seven and a half, distance 5", p. 200 deg., colors white and
purple. This is one of the comparatively small number of stars the
measure of