But parts of the circuit are incomplete, while in some cases dark regions in the very midst of a bright stream attest
the irregular conformation of this strange star region.
Some of the fixed stars are subject to periodical variations of brilliancy. Several, formerly distinguished by their splendour, have entirely disappeared; others are now conspicuous which do not seem to hava been visible to the ancient observers ; and some alternately appear and disappear, or at least undergo great periodic variations in brightness. Some seem to become gradually more obscure, as d Ursas Majoris ; others, like b in the Whale, to be increasing in brilliancy. Some stars have all at once blazed forth with great splendour, and, after a gradual diminution of their light, have again become extinct. The most remark able instance of this sort is that of the star which appeared in 1572, in the time of Tycho. It suddenly shone forth in the constellation Cassiopeia with a splendour exceeding that of stars of the first magnitude, or even Jupiter and Venus at their least distances from the earth, and could be seen with the naked eye on the meridian in full day. Its brilliancy gradually diminished from the time of its first appearance, and at the end of sixteen months it entirely disappeared, and has never been seen since. During the whole time of its apparition, its place in the heavens remained unaltered, and it had no annual parallax ; its distance was consequently of the same order as that of the fixed stars. Its colour, however, underwent considerable variations. Tycho describes it as having been at first of a bright white ; afterwards of a reddish yellow, like Mars or Aldebaran; and lastly, of a leaden white, like Saturn. Another instance of the same kind was observed in 1604, when a star of the first magnitude suddenly appeared in the right foot of Ophiuchus. It presented phenomena analogous to the former, and disappeared in like manner after some months.
Many stars have been observed whose light seems to undergo a regular periodic increase and diminution, and these are properly called variable stars. The star o Ceti (called also Mira) has a period of 334 days, and is remark able for the magnitude of its variations. From being a star of the second magnitude, it becomes so dim as to be seen with difficulty through powerful telescopes. But its maxima and minima of brightness are variable. Thus between October 1G72 and December 16 70 Mira was never visible to the naked eye, while on October 5, 1839, it was half a magnitude brighter than its usual maximum.
Some are remarkable for the shortness of the period of their variation.
Algol ((3 Persei) shines ordinarily as a second magni tude star, but during about 7 hours in each successive period of 69 hours undergoes the following changes : It decreases gradually to the fourth magnitude, remaining so for 20 minutes, and then increases as gradually to the second magnitude. Thus it remains a second magnitude star for about 62 hours in each period of 69 hours. The star /? Lyras in successive periods of 12 days 22 hours undergoes a double change, each occupying 6 days 11 hours (formerly assigned as the star s true period). In each period the star has a maximum brightness of about the 3 5 magnitude, but the minima of brightness are appreciably unequal, the star s magnitude in one being 4 3, in the other 4 5. The star 8 Cephei varies in a period of 5 days 8 hours 48 mm. from the fifth to the 3 5 magnitude, occupying 1 day 14 hours in passing from minimum to maximum, but 3 days 19 hours in passing from maximum to mininum. Two or three hundred stars are now recog nised as variable. The most probable explanation of the variation in long periods is that stars so affected have periodical maxima and minima of disturbance, resembling that which causes the sun spots, only more marked. Some of the variables of short period are probably covered unequally in different parts of their surface by spots. But the varia tion of such stars as Algol, which shine with a constant lustre for the greater part of each period, and are reduced in lustre for a short interval, cannot be thus explained, and we must suppose that a large opaque orb circling around them transits the central luminary at regular intervals. Most of the stars which have appeared suddenly, and shone for a short time, probably resemble in character the only star of this kind hitherto examined the star T in Corona Borealis, which, blazing out suddenly as a second magnitude star (usually of the tenth magnitude only), was found by Hug- gins and Miller to have a spectrum crossed by fine black lines, on which was superposed the spectrum of glowing hydrogen. As the star waned, the hydrogen spectrum faded, and finally disappeared. Presumably there had been an outburst of glowing hydrogen, or a conflagration in which hydrogen was the principal agent. The maximum brightness of the star exceeded the normal lustre almost 800-fold. The star rj Argus can hardly as yet be classed either among periodically variable stars or among stars undergoing irregular changes. In 1677 Halley catalogued it as of the fourth magnitude; Lacaille, ia 1751, observed it as of the second magnitude. Between 1811 and 1815 it was of the fourth magnitude; from 1822 to 1826 of the second ; in 1827 of the first magnitude ; thereafter, till 1837, of the second; in 1838 of the first; in 1843 the star was brighter than any except Sirius. At present it is barely visible to the naked eye.
we find that Halley was the first to suspect from observation the proper motion of the stars. From comparisons between the observed places of Arcturus, Aldebaran, and Sirius, and the places assigned to these stars by the Alexandrian astronomers, he was led to the opinion that all three are moving towards the south. (Phil. Trans., 1718.) The elder Cassiui adopted the same view respecting Arc turus, proving, in fact, that even since the time of Tycho Brahe this star had shifted 5 in latitude ; for he showed that r] Bootis, which should have shared in the change if the ecliptic itself had shifted in position, had not changed appreciably in position. Bradley and Wright theorised respecting the causes of stellar motion, and suggested that the sun and his fellow-suns, the stars, are moving in space. Tobias Mayer, in 1771, comparing Roemer s observations in 1706 with his own and Lacaille s in 1750-56, was led to the opinion that the stellar motions afford no sufficient evidence of the translation of the solar system through space. But in 1783 Sir W. Herschel, from the proper Proper motions of seven principal stars, as determined by Maskelyne, motion of deduced as the point towards which the sun is moving tl)C sun - (or, as it is now commonly termed, "the apex of the solar way") a point in Hercules in right ascension 257. After carefully examining Mayer s list of proper motions, ho indicated a point close to the star X Herculis (Jff,, fig. 52). In 1805 Sir W. Herschel published a paper based on Maskelyne s catalogue of the proper motions of 36 stars published in 1790, and presenting a result consider ably different from that which he had before announced. He now set the apex in R.A. 245 52 30" and north declination 49 38 (}#., in fig. 52). Bessel in 1818 (Fun- damenta Astronomic) expressed the opinion that Mayer had been right in denying the existence of sufficient evi dence for determining the proper motion of the solar system. Later, however, the subject has been dealt with by Miidler, Argelauder, and O. Struve, whose various determinations of the solar apex are shown in fig. 52, marked M, Ar., and 2 respectively. Sir G. Airy re-examined the subject by a new method, assigning to the sun (by an appli
cation of the method ef least squares) such a direction