with Remarks on the Construction of its different Parts . 123 
was kept at some considerable distance by an interval of com- 
parative darkness ; and from its concentric figure I call this 
light an envelope. 
The Figure , Colour , and Magnitude of the Envelope. 
On viewing this envelope in telescopes that magnify no 
more than about 16 times, or in finders and night glasses with 
still lower powers, I found that its shape, as far as it extended, 
was apparently circular ; but that in its course it did not reach 
quite half w r ay round the head of the comet. A little before 
it came so far it divided itself into two streams, one passing 
by each side of the head. 
The colour of the envelope in my 7, 10, and 20 feet tele- 
scopes had a strong yellowish cast, and formed a striking 
contrast with the greenish tint of the head. 
The distance of the outside of the envelope from the centre 
of the head, in the direction of a line drawn from it to the 
sun, was about 3' 30"; and supposing it to have extended 
sideways, without increase of distance as far as a semi-circle, 
this would give its diameter about 19 minutes. By computa- 
tion therefore its real diameter must have exceeded 643 thou- 
sand miles. 
The Tail of the Comet. 
The most brilliant phenomenon that accompanies a comet 
is the stream of light which we call the tail. Its length is well 
known to be variable, but the measures or estimations of its 
extent cannot be expected to be very consistent from several 
causes foreign to its actual change. 
The 2d of September, the moon being up, the comet very 
low, and the atmosphere hazy, I could perceive no tail. 
R 2 
