122 
Dr. Herschei/s Observations of a Comet , 
A transparent and elastic Atmosphere about the Head. 
In every instrument through which I have examined the 
comet, I perceived a comparatively very faint or rather darkish 
interval surrounding the head, wherein the gradually dimi- 
nishing light of the central brightness was lost. This can 
only be accounted for by admitting a transparent elastic atmo- 
sphere to envelope the head of the comet. 
Its transparency I had an opportunity of ascertaining the 
18th of September, when I saw three very small stars of dif- 
ferent magnitudes within the compass of it ; and its elasticity 
may be inferred from the circular form under which it was 
always seen. For being surrounded by a certain bright equi- 
distant envelope, we can only account for the equality of the 
distance by admitting the interval between the envelope and 
the head of the comet to be filled with an elastic atmospherical 
fluid. 
The Extent of the cometic Atmosphere. 
When I examined the comet in the 20 feet telescope the 
6th of October, the circular darkish space, which surrounded 
the brightness, just filled the field of the eye-glass; which 
gives its apparent diameter 15 minutes. This atmosphere was 
therefore more than 507 thousand miles in diameter ; but its 
real extent of which, as will be seen, we can have no obser- 
vation, must far exceed the above calculated dimensions. 
The bright Envelope of the cometic Atmosphere. 
When I observed the comet at Alnwick in an achromatic 
refractor with a magnifying power of 65, 1 perceived that the 
Stead of it was partly surrounded by a train of light, which 
