REVIEWS—DONATI’S COMET, 61 
its outline lost in mist at the end: and to these three, we must now 
add the veil, first observed in the present comet, a dim band of light 
surrounding the head and tail, sharply distinct from them by the 
faintness of its illumination. These characteristics, however, do not 
mark every comet. Many of them have no trace of tail: one, at least, 
has been recorded whose head entirely disappeared, leaving a long tail 
as its only representative: others have had more than one tail,—that 
of 1744 had six, “spread out like the sticks of a fan,’’—in several, an 
additional tail has been thrown out towards the sun,—in other cases the 
twin tails have been inclined at angles from 18° to 120°, and some- 
times there have been streamers darted out like those of the Aurora: 
in the present comet, two such were seen ; one of them starting from 
the head and touching the tail proper, running in a nearly straight 
line far beyond it, and the other shooting out from the tail itself about 
one third of its length from the head, and running off ata different 
angle from the former. Again with regard to the nucleus, it has been 
totally wanting sometimes, and both it and the planet-like disc shewn 
in other cases have resolved themselves under a high power of the 
telescope into nebulosity ; on the other hand, the head has in two or 
three cases appeared to consist of a number of such stellar points ; 
Halley’s comet in 1835 was seen by Sir John Herschel at the Cape to 
have formed a new nucleus after its perihelion passage, and this 
nucleus had a diminutive coma and tail within and distinct from the 
original head, a miniature comet within a comet; the like phenomena 
having also being long ago recorded by Kepler. But the most wonder- 
ful vagary of this kind was that actually seen in Biela’s comet, which 
beneath the observer's eyes, split itself into two distinct comets, each 
having its own appendages complete, and travelling side by side the 
rest. of their course with achain of light uniting them. After undergo- 
ing various alternations of relative brightness, the old one appeared at 
length to obliterate its companion, and threw out three tails as if in 
token of victory, but the distance between them was great enough to 
render it probable they would henceforward move as independent 
comets, and their return has been anxiously looked for, though as, yet. 
(we believe) without success. 
It is, however, in the tail that the great mystery lies. We have men- 
tioned that the tail (speaking loosely,) is always directed from the 
sun: that is, it follows the comet in its descent to the sun, and pre- 
cedes it in its recess. Now if the tail be a material body, acted on 
