An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

295 lclXEi.) STA' 8.
inragmitude. It then decreases about three monthls, ulnt.il it becoimes completely invisible, and remains so about five montlh,
whcll it again becomes visible, and continues increasing duringill
the remaining three mlllthls of its pleriod.
Another very remarkable variable star is Algol (3 Jcrsaci).
it is suddenly visible as a star of the second magnitude, and
contilues such for 2d. -ltl., wheln it begins rapidly to dinninishl
in splendor, andt in ab)out 3- hours l is reduced to the fourtht
1imagnitude. Jt then begins again to increase, and inl 3 ll hours
mnore, is restored to its ulsual brighltllness, goinlg through all its
cllhangcs in less titan tlhree days.'hllis remarkable laNw of varnliatioll appears strongly to suggest the revolution round it of
some11 opaque body, which, when intcerposed between us and
Alol, cuts off a large portion of its ligllt. Tt is (8'sa1ys Sir
J. Hlerschel] ) all indicationl of a highl degree of activity in regions
wherel but for such evidence, we migl!t conclude all to be lifeless. Our su1n requires almost IlilnC times this period to 1)erfolrl
-a revolution on its axis. On tlhe othecr hand, the periodic time
of anl opaque revolving body, sufficiently large, whic \would
produce a similar tclmn)oary obscuration of the sun, seew fri'Cn
a fixed star, Awould be less thlan fourteen hoursl.
Tice duration of tcese Iperiodls is extremely various. While
that of P3 1erscei, above mentioned, is less tilan three days,
others are more than a year, and otshers many yearls.
42<4. eXWuS'TfrltS AND NPtRhliAt.tii various parts of the firmament are seen large groups or
CLU.tus'rls, which, either by thle naked eye, or by the aid of the
smallcest telescope, are perceived to consist of a great number
of small stars. Such are the l.'leiades, Coma iBerlcliess, and
P1?rselpe, or the Beelhive in Cancer. 1The PIleiades, or S.ve,t
kSths, as thtey are called, in thlie neck of Taurus, is the most
consl)icuolus clusterI. Wheln Awe look dirictly at this gro0up, w\fe
can not d(istinguishl more than six stars, but by turning thle yes
a little to one side,X \o discovlter that there are many more.
W itndirect vision is fair more delictate than (li-rcot. Thus we can see tihe Zodiacal,ight o(r a Comet's tail much )mlore distinctly and of greater length, if, instead
of looking directly at it, we turn the eyes to vaqrious points ne-rw} it, tle a!tea!iot all
thle while being givel to thle object. itselt.