204 On the Great Comet of 1843. 
tion is made only of the elastic force of the vaporous matter sur- 
rounding the comet and composing its envelope or tail. On this 
hypothesis we have suggested the simplest and most natural meth- 
od of completing the elliptic elements, viz. that of making the 
excess of the supposed elliptic over the actual hyperbolic radius 
vector of the point observed equal to a constant co-efficient of the 
reciprocal of the square of the radius vector, and determining by 
means of the constancy of this value, the actual eccentricity cor- 
responding to a period of 21% years, and a perihelion point of the 
centre of gravity or head of the comet actually outside of the 
sun though nearly in contact with it. In fact this multiplier is 
not necessarily constant, nor necessarily a co-efficient of the re- 
ciprocal of the square of the radius vector; still this hypothesis 
is the most simple and plausible that can be made, and is perhaps 
quite as complex as the nature of the question permits us to make. 
As the subject of the physical organization of the head, tail, 
and nebulosity or envelope of comets, has been discussed by Sir 
William Herschel, Olbers, Brandes and Bessel, with their charac- 
teristic genius and acumen, we deem it proper to consider the 
bearing of their opinions and researches on the present question. 
Sir William Herschel* states that the kernel or head of the 
sreat comet of 1811, of about 1” in diameter, could only be seen 
with high powers in his most powerful telescopes, and that with 
ordinary instruments he saw only the nebulosity or envelope; 
but that when the head or kernel was seen, it was seen within 
the envelope eccentric from the sun, or in other words the densest 
portion of the envelope or nebulosity was eccentric towards the 
sun. ‘This is precisely the phenomenon observed by Messrs. Her- 
rick and Bradley with reference to the disc of the nebulosity, 
though the kernel or head could not be seen. ‘This also agrees 
with Bessel’s remark, that this comet seems to have thrown out 
nearly all its head in forming the nebulosity and tail. 
We come next to Olbers’s theory of the formation of the en- 
velope and tail of comets. This was promulgated in 1812, short- 
ly after the appearance of the great comet of 1811. We do not 
recollect to have any where met with a translation of it. It is 
perhaps the only theory ever proposed that explains all the phe- 
* Monatliche Correspondenz, Vol. xxvii, p. 459. 
t Ibid., Vol. xxv, p. 3. 
