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

CO0 MIqrs. 2
sun;an this, as that of 1811 must have receded from it more
than 4500,000,000 miles, while sollme even are spo to
pass into parabolic or 1hyperbolic orbits, and never to return.
l394. Comets shine by relectih? t1e iflht of the n. I
or o instances tlhey ]have exhibited distinctphas, althouh
the nelulou matter with whlich the nucleus1 issu nded
oull Commonly 1)revent such Plhases from being distinctly
visible eve whe tley would otherwise be apparent. Moreo, certain qualities of l)olarizcd light enable the optician to
decide whther thle liglht of a given body is (direct or reflected
and M. Arago, of ]Paris, by experiments of this ind o the
of the comect of 1819, ascertained it to be reflected ight.
Te tail of a comet; usually increases very much as it approachs tl sun5.; and f'requently does not reach its maximm
ntil after te perilelion passage. 1n receding from the sun
the tail agam contracts, and nearly or quite disappears befor
the body or the comet is entirely out of sight. The tail is fquentl (ivided into two portioIns, the ccntral pats, in the
direction of;e axis, being less br1iitht t1han tile, l1nr na lrls.
InI 1.741, a comect appeared which had six tails, spread out like
a fan.
The tails- of comets extend in a. direct, line from the suni, although theyr are usually more or less curved, like a longr quill
or feather, being,, convex onl the -side next, to the direction in
whichtheyare moving (Ficr. 7(1); a figr which may res-lmlt
from thec less velocitye) of the portions most remnote. from thec
sunl. ExpaInsions of the envelope haive -also been at times
observed onl the side next the sun,:1"t but these, seldom attainl
any considerable length.
39i -'ihie qantity f meatter in comets is exceedingly small.'Their tails consist of' matter of such tenuity that thle sma-llest
stars~ are visible through them. TheI-]iy can only be regrardled
as, great masses of thinl vapor, susceptible of belcin penetrated
through their whole substance by thle sunbeams, and reflecting
them alike from their interior, parts and from their surfaces.
oIDclibe.ii, 400. f imxm.151.
Sicc Dr..Thslin'ts I'Cemark$ on Halley's comet, Amer. Journ. Science, xxxil.