n the order of their relative brightness. Thus if for a group of eight
stars we have found at one epoch A, B, C, D, E, F, G, H, and if at
another time the order was A, B, C, D, F, E, G, H, symptoms of
variability are pointed out. Repeated observations, where the same star
is found in different sequences, will decide the question. Thus, for the
stars visible to the naked eye, we know exactly the state of the sky in
HERSCHEL'S day, now nearly a century ago. Any material change cannot
escape us. These catalogues have been singularly overlooked by the
observers of our generation who have followed this branch of
observation, and it was not till 1876 that they received proper
attention and a suitable reduction (at the hands of Mr. C. S. PIERCE).

We owe to HERSCHEL the first trustworthy account of the stars visible
to the naked eye, and since the date of his labors (about 1800) we have
similar views published by ARGELANDER (1839), HEIS (1848), ARGELANDER
and SCHOeNFELD (1857), GOULD (1860 and 1872), and HOUZEAU (1875). Thus
his labors have been well followed up.

In the prosecution of this work HERSCHEL found stars whose light was
progressively diminishing, others which regularly increased, one star
whose light periodically varies (_[alpha] Herculis_), and at least
one star (55 _Herculis_) which has utterly disappeared. On October 10,
1781, and April 11, 1782, he observed this latter star, but in May,
1791, it had totally vanished. There was no trace remaining.

The discovery of the variability of _[alpha] Herculis_ was a more
important one than would at first sight appear. Up to that time the only
variable stars known were seven in number. Their periods were four
hundred and ninety-four, four hundred and four, three hundred and
thirty-four, seven, six, five, and three days. These periods seemed to
fall into two groups, one of from three hundred to five hundred days,
the other comparatively much shorter, of three to seven days.
_[alpha] Herculis_ came to occupy the middle place between these groups,
its period being about sixty days.

The cause of these strange and regular variations of brightness was
supposed by HERSCHEL to be the rotation of the star bodily on an axis,
by which revolution different parts of its surface, of different
brilliancy, were successively and periodically presented to us. This
explanation it might have been difficult to receive, when the periods of
the known variables were so markedly various in l