68 REVIEWS—DONATI’S COMET. 
of red-hot iron.* But by so fierce a heat, vapours and exhalations, 
and every volatile matter must have been immediately consumed and 
dissipated.” 
At the present day we should draw a conclusion from the same fact 
directly opposite to Newton’s. We can hardly conceive that a solid 
body would zot be vaporised by so fierce a heat. The question of 
opacity would of course be settled at once by the occultation of a star 
behind the nucleus, and there seem to have been plenty of cases where 
this would have been seen if it had occurred, yet Bessel says that he 
cannot satisfy himself that any observed passage of a comet over a 
star has been really central. Whether, however, there be any solid 
nucleus or not, the tail and head must be of extreme tenuity, for stars 
have been repeatedly seen through all parts, in some cases with bril- 
liancy unaltered, in others, diminished, (as was to be expected, partly 
from the p2rspective effect, partly from the absorption of light by the 
passage,) and in a few even increased. This last result is so odd that _ 
one would like to disbelieve it, but the evidence seems too strong, for 
Relhuber in a comet of 1846 says that a star of the 8th magnitude 
(just invisible to the naked eye) when it was centrally covered by 
the comet, became very considerably brighter and was judged to be 
equivalent to one of the 6th, in which case it could have been distinetly 
seen without a telescope.t An opaque body also, if not self-luminous 
*This remarkable calculation—almost a divination for the time it was made—has been re= 
peatedly but imperfectly quoted, and almost always misunderstood. Arago has been evident- 
ly misled, by one of these imperfect references, to make an objection to it which is altogether 
mistaken. The following is the original passage :—“ Est enim calor solis uf radiorum den- 
sitas, hoc est, reciproce ut quadratum distantiz locorum a sole. Ideoque cum distantia 
cometis a centro solis Decemb. 8, ubi in perihelio versabatur esset ad distantiam terre a 
centro solis ut 6 ad 1000 circiter, calor solis apud cometam eo tempore erat ad calorem solis 
gestivi apud nos ut 1600000 ad 36, seu 28000 ad 1. Sed calor aque ebullientis est quasi triplo 
major quam calor quem terra arida concipit ad astivum solem, ut expertus sum, et calor ferri 
candentis (si recte conjector) quasi triplo vel quadruplo major quam calor aque ebullientis ; 
ideoque caior, quem terra arida apud cometam in perihelio versantem ex radiis solaribus 
concipere posset quasi 2000 vicibus major quam calor ferri candentis.” 
+ Quoted in Hind’s Comets. We add the following from the present volume, “ When it 
{Arcturus] had entered well within the margin of the tail, a dark notch was formed, entting 
out a portion of the tail round the star ; and as the star got farther in, this became a dark 
aureola surrounding the star, and in diameter equal to about one tenth of the line of transit. 
This continued until the star reached the middle; at this part there is a broad dark line 
which extends from the nucleus toa distance considerably above the point where the star 
crossed. When Arcturus arrived here, this dark space was perfect up to the star, but on 
the other side the white light of the tail appeared to come quite up to the star ; in short, as 
the bright part of the tail had been darkened in the vicinity of the star, the dark part was 
brightened, at least so much of it as was on the side farthest from the nucleus. I saw the 
notch‘again on the opposite side previous to emersion, and then lost it by clouds. The ef- 
fects I have described are, doubtless, optical, and the notch and aureola evidently due to the 
bright light cf this star ; the effect on the dark central part is not so easy to explain.” 
