138 Dr. Herschei/s Observations of a Comet , 
deed, how far it may extend beyond the region which contained 
the shining substance, we can have no observation to ascer- 
tain, on account of its transparency. In consequence of the 
darkish interval, occasioned by the atmospheric space, the 
suspended light appeared to us in the shape of a very bright 
envelope. 
The brilliancy of the envelope, and its yellowish colour, so 
different from that of the head, and probably acquired by its 
mixture with the atmospheric fluid, are proofs of the continued 
action of the sun upon the luminous matter, already in so high 
a state of rarefaction ; and if we suppose the attenuation and 
decomposition of this matter to be carried on till its particles 
are sufficiently minute to receive a slow motion from the im- 
pulse of the solar beams, then will they grad ually* recede 
from the hemisphere exposed to the sun, and ascend in a 
very moderately diverging direction towards the regions of 
the fixed stars. 
That some such operation must have been carried on, is 
pretty evident from our having seen the gradual rise, and in- 
creased expansion of the tail of the comet ; and if we saw the 
shining matter, while suspended in the cometic atmosphere, 
in the shape of an envelope, it follows that, in its rising con- 
dition, it would assume the appearance of those two luminous 
branches which we have so long observed to inclose the tail 
of the comet. 
The seemingly circular form, and the stream-like appear- 
ance of the luminous matter having been already explained, 
we may now see the reason why it can rise in no other form 
than the conical ; for a whole hemisphere of it being exposed 
to the action of the sun, it must of course ascend equally every 
where all around it. 
