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PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 135 
merely so much the more difficult to distinguish, by reason of the well-known optical or physiological 
law, that a small difference in the brightness of two objects is more difficult to perceive in proportion 
to their absolute brightness. If our sensation and means of measurement are not sufficiently accurate 
in this case of the thin cloud, we have only to turn to a thicker cloud (of the same species, and in a 
similar part of the sky with regard to the sun), and there we shall see the same law which must 
obtain in the former case now visibly developed ; and then we come to the necessary conclusion, that 
the illuminated side of every cloud must be brighter than the other, i. ¢., that it must,shew some 
hase. 
The comets are undoubtedly far rarer than any description of cloud floating in our atmosphere, 
but they are seen under far more favourable circumstances for exhibiting a phase; for, they are 
illuminated by the sun from one end, so that there must be a much greater difference of intensity of 
light at the two ends than there would be at the two sides, if transversely lighted, as with the long 
thin films of bright cloud alluded to; and, further, the comet being of the last degree of faintness, tke 
eye is much better able to detect einll differences of luminosity. Then again, comets, though they 
be exceedingly rare, are very voluminous, so that the rays of light have to traverse a great space of 
matter in passing through them; and if some is reflected in the anterior parts of the body, as we 
see is the case by the fact that the body is rendered visible to us, there cannot possibly be so 
strong an illumination on the posterior parts ; therefore, we shall either see them fainter than the 
others ; or not at all, if the anterior portions themselves are but just visible. 
With these preliminaries then, we may ask, what comet has ever been seen without some phase ? 
for in every single instance, the anterior part of the head, or the denser portion of the envelope, has 
been brighter than the posterior, exhibiting sometimes the appearance of a luminous sector in front ; 
and the anterior half only, of the body, has been seen, the comet presenting as a general rule two 
diverging and slightly curved tails. This has been generally held to be merely the effect of looking 
transversely through a conical envelope of luminous matter, when the ray of light passing through the 
central portions would meet with less substance, and that part would therefore appear darker than 
the limbs. This, doubtless, prevails to a great extent, but then we must further remember, that the 
exterior coats of the envelope will be more strongly illuminated than the interior ; and the dark axis 
of the comet’s tail becomes therefore a particular character of phase. Further, as we procced to the 
posterior portions of even the outer coats of the envelope, they will be ieeniented by a weaker light 
from the sun, by reason of their greater distance; and if any convergence of them towards the axis 
should occur, as has actually been observed in some cases, their illuminating rays being then still 
further diminished in intensity by absorption and reflection, they will hardly be enabled to make them- 
selves visible to us. Thus, the diverging limbs of the tail, and its forked or many-pointed termination, 
becomes an effect of phase on a body which may be of asymmetrical and rounded, and complete character. 
This point, it is of the greatest importance to determine, for if the actual forms of comets be as 
we see them, they are altogether anomalous in the heavenly regions ; and merely on the score of the 
form of these supposed conical envelopes and diverging streamers, equally anomalous forces have been 
introduced to explain the phenomena; electricity and polarity, which have no place in any ae 
department of astronomy, being allowed precedence here. 
_ Granting, that a comet is always a prolate spheroidal mass of vapour of different degrees of 
prolateness, and of actual length in various cases, but always illumined from one end, then we may 
expect in the larger and denser comets to see but the anterior half of the body; the posterior half 
being so much further off from the sun, and the rays of light which reach it, being further so much 
weakened by having passed through the first half; consequently, in this description of comet, we 
might expect, and we absolutely do find, the phenomenon of the forked tail most marked. In the 
smaller and fainter comets, on the other hand, the rays of light which reach the posterior half of the 
body are not much dimmed either from having passed through the excessively tenuous anterior por- 
tion, or from having travelled through any notably greater distance from the centre of radiation; in 
such cases we may expect to see more completely the whole form of the comet; and in them we do 
actually find nearly, and sometimes quite oval forms, and all gradations from these, through truncated 
ovals to the forked tails. 
These facts induce us to admit the possibility of the bodies of comets being of « far more regular 
geometric form than has hitherto been suspected, if we allow that conclusions derived from small 
comets may be safely so extended, mutatis mutandis, to large ones; but this view is further confirmed 
by a notable observation of one of the largest and most excentric of comets. 
As already observed, comets decrease in size and inerease in density on approaching the peri- 
helion, and the reverse on receding therefrom ; hence the phase ought to be most evident, or the tail 
VOL. XX. PART I. 20 
