REVIEWS—DONATI’S COMET. 65 
which Prof. Bond may claim the lion’s share of credit, relates to the 
knowledge he has been enabled to gain of the formation of the tail 
itself. 
Sir William Herschel was the first to notice in the comet of 1811, 
that the tail seemed to be only a prolongation of part of the coma 
which was separated from the head by a dark interval, and was led to 
the inference that the tail was in effect a hollow envelope enclosing 
the head at its upper part, and having a space between it and the 
head occupied by some dark atmosphere or non-luminous gas, thus 
satisfactorily accounting for the brightness of the apparent edges and 
the obscurity of the central part or dark zone. In the comet of 
1835, this inference was confirmed by the experience of Bessel and 
others, with the addition that this envelope was connected with the 
head by a conical jet of light, an azgrette lumineuse or luminous sec- 
tor, which proceeded in a fan-like shape from the nucleus toward the 
sun and then bending back on both sides seemed to send a flow of 
luminous matter into the tail. This jet was by no means stable in 
form; sometimes single, sometimes split up into several ; now thin, 
now broad, but always brighter at its start from the nucleus and 
gradually melting into the haze of the coma. Bessel added the 
curious fact that the axis of this aigrette was in a state of rapid and 
continuous oscillation about an axis perpendicular to the plane of the 
orbit, never deviating far from its mean position, nearly directed to- 
wards the sun. This has not been observed in recent cases; the 
comet of 1858 showed no symptoms of such a movement. So far 
then we should infer that the tail was formed by a stream of luminous 
matter projected in a conical jet from the nucleus towards the sun, and 
then meeting with some repulsive agency, was checked, and turning back 
flowed round the sides in a continuous stream with ever-widening 
section. Sir John Herschel very aptly compares it to the trail that 
follows the smoker against a brisk wind. But the admirable examin- 
ation of our 1858 comet by Prof. Bond leads us to modify this con- 
clusion. He has shown that the tail is not thus formed by a continuous 
jet, but is due to a series of envelopes which are succcssively thrown 
off. like skull-caps, from the head. No less than five of these were 
identified and consecutively watched, (in that of 1861, there were no 
less than eleven) and so well did Prof. Bond become acquainted with 
the habits of his patient, that he was able to predict the recurrence 
of the event. 
Vou. VIII. F 
