On the Great Comet of 1843. 207 
orbital direction and velocity of the particle at the time of expul- 
sion. ‘The reason assigned by Bessel for not supposing a mole- 
cular connection between the particles of the nebulosity of the 
comet, is the fact that the rays from a star seen by himself in 
the Konigsberg heliometer, through this nebulosity, were not re- 
fracted by the medium of the nebulosity, but preserved, when 
seen through this medium, the same relative position with refer- 
ence to any other fixed star as when seen before entering the 
medium. Bessel argues that this could not have been a vapor, 
gas, or air, through which the star was seen, or it would have 
been refracted. 
Having stated the principal points of the theories of Olbers and 
Bessel regarding the formation of comets’ tails, it remains to con- 
sider their bearing on the present question. It is obvious that the 
difference between the radius vector of the head and of any particle 
(one in the point aimed at for instance) of the nebulosity, would 
not be a simple multiplier of the reciprocal of the square of the 
radius vector; but would on the contrary include a term depending 
upon the reciprocal of the square of the distance of this particle 
from the centre of the head. It may be shown, however, from 
the length and narrowness of the tail of the recent comet, that 
this latter term is nearly insensible, since the comet’s expulsive 
force compared with the sun’s, must in this instance have been 
very small, for when acting in the normal to the radius vector, it 
was able to impress but a small normal velocity on the particles 
thrown off. 
We conclude then that whatever theory of the formation of 
comets’ tails we adopt, if we suppose the particles of the nebu- 
losity and tail to be material, and the densest portion of the for- 
mer to be extended from the centre of the comet’s head towards 
the sun, the simplest method of deriving the true elements of the 
orbit of the head, from the computed elements of the orbit of the 
point observed, is that which we mentioned in our letter of the 
25th of May last. 
We subjoin the elements of the comet computed from our first 
normal places by Prof. Anderson of Columbia College, New York, 
viz. 
