70 REVIEWS—DONATI'§ COMET. 
«comet was broken up by the impact of the solar rays, and that the 
light particles were carried away by the impulse, forming the tail, 
while the denser ones stayed behind and made the head; and this was 
_an ingenious conjecture, the Solar rays being imagined as something 
shot out from the sun like arrows. Newton was the first to advance 
.a better founded theory. He supposed the existence of an ether per- 
wading space or an atmosphere of the sun, and that the parts of this 
in the vicinity of the comet, becoming rarified by the heat which it 
acquired from its approach to the sun, ascended amid the cooler and 
denser atmosphere, carrying off with it the luminous particles of the 
comet, just as we see in our atmosphere a current of heated air ascend, 
carrying up with it the smoke of a fire. This hypothesis goes a long 
way toward satisfactorily explaining the prominent phenomena con- 
cerned, such as the form of the tail, its curvature, the deviation of its 
axis, the brightness of the forward edge, but it fails to account for the 
‘more unusual ones, such as the secondary tails. And perhaps the 
‘strongest objection to it is that which lies on the surface, namely, the 
sabsence of proof that there exists such a vera causa as the assumed 
ether. It is true that observations on Encke’s comet establish a 
-gradual diminution of its periodic time, an effect which would be pro- 
duced by the resistance of such an ether, (and indeed such an effect 
was predicted by Newton himself, whose sagacity nothing seems to 
have escaped, for the comet of 1680, a prediction not fulfilled how- 
ever,) but it cannot be held established that this is the very cause 
which produces the said effect. For the calculation of the retardation 
in this single case would only enable us to ascertain the law of resist- 
ance of the ether assumed to exist, and it would be necessary to show 
that this law satisfies also the retardations in other observed cases, be- 
fore we can assert the truth of our hypothesis, and no such other case 
has yet been found, which fact is itself almost conclusive against the 
hypothesis. 
Bessel, reasoning from the appearances presented in his observation 
of Halley’s comet, has developed another theory which is waiting the 
test of facts for acceptance or rejection. The conical jet, or azgrette 
lumineuse, had a rapid oscillatory movement from one side to the other 
of the sun’s radius-vector. According to Bessel this can not be ex- 
plained as an effect ot the attraction of the sun. For although the 
attraction might cause an oscillation (corresponding to the hbration 
of the moon), the period of it would be very long, while the observed 
duration was very short. It is necessary therefore to infer that the 
