es its light to affect the retina of the eye less quickly
than does that of its brighter companion, and, in consequence, the
reversal of its apparent motion with the swinging of the telescope is
not perceived so soon.

The third-magnitude star eta has a companion of magnitude ten and a
half, distance 90", p. 340 deg.. The star beta, of the second magnitude, and
reddish, is variable to the extent of half a magnitude in an irregular
period, and gamma, of magnitude two and a half, has an
eleventh-magnitude companion, distance 162", p. 285 deg..

[Illustration: MAP NO. 20.]

Our interest is revived on turning, with the guidance of map No. 20,
from the comparative poverty of Pegasus to the spacious constellation
Cetus. The first double star that we meet in this constellation is 26,
whose components are of magnitudes six and nine, distance 16.4", p.
252 deg.; colors, topaz and lilac. Not far away is the closer double 42,
composed of a sixth and a seventh magnitude star, distance 1.25", p.
350 deg.. The four-inch is capable of splitting this star, but we shall do
better to use the five-inch. In passing we may glance at the
tenth-magnitude companion to eta, distance 225", p. 304 deg.. Another wide
pair is found in zeta, magnitudes three and nine, distance 185", p. 40 deg..

The next step brings us to the wonderful variable omicron, or Mira,
whose changes have been watched for three centuries, the first observer
of the variability of the star having been David Fabricius in 1596. Not
only is the range of variability very great, but the period is
remarkably irregular. In the time of Hevelius, Mira was once invisible
for four years. When brightest, the star is of about the second
magnitude, and when faintest, of the ninth magnitude, but at maximum it
seldom exhibits the greatest brilliance that it has on a few occasions
shown itself capable of attaining. Ordinarily it begins to fade after
reaching the fourth or fifth magnitude. The period averages about three
hundred and thirty-one days, but is irregularly variable to the extent
of twenty-five days. Its color is red, and its spectrum shows bright
lines, which it is believed disappear when the star sinks to a minimum.
Among the various theories proposed to account for such changes as these
the most probable appears to be that which ascribes them to some cause
analogous to that operating in the production of sun spots. The
outburst of light, however, as pointed out by Scheiner, sh