136 Dr. Herschei/s Observations of a Comet , 
an hemispherical hollow shell, the luminous matter in its di- 
verging progress upwards can only form a hollow cone ; and 
the appearance of the two bright streams inclosing the tail, 
after what has been said of the envelope, will want no farther 
explanation. 
Add to this that, having actually seen these brilliant streams 
remain at the borders of the tail in the same diverging situa- 
tion during a motion of the comet through more than 130 
degrees, the hollow conical form of the comet’s tail is in fact 
established by observation. 
The feebler light of the tail between its branches is suffi- 
ciently accounted for by the thinness of the luminous matter 
of the hollow cone through which we look towards the middle 
of the tail compared with its great depth about the sides ; and 
indeed the comparative darkness of the inside of the cone and 
transparency of the atmosphere seen through the envelope, 
bear witness to their hollow construction ; for, were these parts 
solid, both the cone and the hemispherical termination of it 
must have been much brighter in the middle than towards the 
circumference, which is contrary to observation. 
Of the solar Agency in the Production of Cometic Phenomena. 
As we are now in a great measure acquainted with the phy- 
sical construction of the different parts of the present comet, 
and have seen many successive alterations that have happened 
in their arrangement, it may possibly be within our reach to 
assign the probable manner in which the action of such agents 
as we are acquainted with lias produced the phenomena we 
have observed. 
