REVIEWS—-DONATIS COMET. 69 
or not surrounded by a luminous atmosphere, would present phases’ 
like the planets, and onthis point again the evidence is conflicting, but 
the advantage lies decidedly on the negative side. 
Is the light of the comet its own? The very sudden changes of 
brilliancy would seem to answer this question in the affirmative, if it 
were not that they may be equally well supposed to arise from sudden 
changes of density in the substance of the comet which might change 
its capacity for reflection; and the same consideration negatives also 
the argument that may be drawn from the observed illumination at 
different distances not conforming to the arithmetical ratio it should. 
follow, if due wholly to the sun. On the other hand, the light from 
the comets has been found in some cases to be polarised, proving that 
some portion at least of it has undergone reflection; but in other 
cases (notably in that of 1843) no trace of polarisation could be detect-- 
ed, and it is just to infer that these comets at least were in the condi- 
tion of an incandescent gas. And again, as Bessel has remarked, if 
the substance of the comet be capable of reflecting light, it must also 
be capable of refracting it, and this would be evidenced by the change 
of position in a star seen through it. A very favorable opportunity of | 
testing this enabled him to assert that there was no such refraction,. 
or at least none large enough to be sensible to our most refined obser- 
vations. And indeed such a refraction could hardly be expected to be 
sensible, when we consider how excessively refined the density of the 
cometic substance must be to occupy such immense spaces with so 
small a mass,— so small indeed that no disturbance has ever been de- 
tected as produced by them in the motions of the least of the planetary 
system, as in the case of Lexell’s comet which paid a visit to Jupiter, 
and so far from deranging his satellites, was itself diverted from its 
proper orbit and sent off to wander anew in distant regions, never 
having been seen among us since. On the whole it seems probable 
that there is no solid substance in a comet, but that it is a mass of 
extremely rarified incandescent vapor, reflecting also the light of the 
sun, and thus shining both by its own and by borrowed light, 
The older philosophers were content to say that the particles of a 
comet’s tail ascended from the sun by virtue of their inherent levity, 
just as some bodies fell to the earth by virtue of their inherent 
gravity. Another hypothesis made the tail to be only the effect of 
light in passing through the nebulosity of the head, like the beam of 
sunlight admitted through a small hole into a darkened chamber, and 
viewed transversely. Kepler conceived that the substance of the 
