40 
its wings and creating a disturbance which was visible 
before the insect which produces it. 
There is nothing else that we can conceive their tails to be 
so that they must be one or other of these two things; either 
(1) Material appendages of the nucleus, whether the 
material be limited to the illuminated tail or surround the 
comet on all sides. 
(2) Matter which exists independently of the comet, and 
on which the comet exerts such a physical influence as to 
render it visible. 
Respecting the composition of these bodies, Sir John 
Herschel says : — “ There is beyond question some profound 
secret and mysteiy of nature concerned in the phenomenon 
of their tails. Perhaps it is not too much to hope that 
future observation, borrowing every aid from rational specu- 
lation, grounded on the progress of physical science generally 
(especially those branches of it which relate to the setherial 
or imponderable elements) may ere long enable us to pene- 
trate this mystery, and to declare, whether it is matter in 
the ordinary acceptation of the term that is projected from 
their heads with such extravagant velocities, and if not 
impelled at least directed in its course by reference to the 
sun as a point of avoidance. In no respect is the question as 
to the materiality of the tail more forcibly pressed on us for 
consideration than in that of the enormous sweep which it 
makes round the sun in perihelio, in the manner of a straight 
and rigid rod, in defiance of the law of gravitation, nay, even 
of the received laws of motion, extending (as we have seen in 
the comets of 1680 and 1843) from near the sun’s surface to 
the earth’s orbit, yet whirled round unbroken : in the latter 
case through an angle of 180’ in little more than two hours. 
It seems utterly incredible that in such a case it is one and 
the same material object which is thus brandished. If 
there could be conceived such a thing as a negative shadoiv t 
a momentary impression made upon the luminiferous tether 
