REVIEWS—DONATI’S COMET. 59 
number even of those which come into our system, ({or it is only of 
such that we have cognisance), not to speak of those outside of it. 
Many there must have been too faint to be visible at all; many of them 
not visible from our situation at the time of their approach to the sun. 
It must have been a startling apparition during the total eclipse of 
62, A.D., when a huge swordlike comet was seen close to the sun, not 
seen before or after. Some few have been bright enough to be seen in 
broad daylight, but not many can bear the light even of the full moon, 
and, in more than one instance, after approaching the sun in consider- 
able brilliancy and being lost for a time in his glare, the comet has 
never reappeared on the other side of him, though closely watched for. 
Very capricious too are even the periodic ones in their spiendor; as 
that of 1759 which, from its decaying lustre and diminished tail at 
each successive appearance, deluded Laplace into a conjecture that it 
might be undergoing a process of condensation and solidification which 
would fit it to become an orderly member of the planetary group, a 
world in the act of manufacture under our eyes; and this would have 
been a triumph for the “nebular Hypothesis.’ Unfortunately, in 
1835 it came round as bright as ever, spreading its tail with more 
than its pristine sweep, and the Nebular had one more disappoint- 
ment to put up with. 
The days are past when 
“Some pilgrim comet, on his way 
To visit distant shrines of light,” 
could throw the nations into panic as a forerunner of plague, war, 
and pestilence, could drive emperors from their thrones and be 
anathematised by Popes, but not less at the present day is the excite- 
ment, though of a more pleasurable kind, caused by these strange 
visitors. ‘‘There is, beyond question,’ says Herschel, ‘some pro- 
found secret and mystery of nature concerned in the phenomenon of 
their tails.’ To fathom this mystery, to trace the history and hidden 
cause of the wonderful changes and disturbance that a comet under- 
goes, is now and has for years been earnestly attempted by astronom- 
ers, and no sooner does a comet swim within our ken, than it is 
watched by hundreds of eager telescopes which dog it with unrelaxing 
attention through every step of its visible course. Nor can a better: 
proof of the keenness of this pursuit be given than is furnished by the 
magnificent volume whose title is cited at the head of these pages. 
We have here brought within one grasp the whole of the observations 
