W. A. Norton on Comets. 97 
following side of the tail was no longer noticed. Another cause 
is in sapere while a comet is receding from the sun, to pro- 
e same results; the central particles, which we haye seen 
observ 
Envelope of the Head of a Comet, as the. Comet approaches and re- 
cedes from the i 
taken place in the length and distinctness of the tail of a comet, 
are merely apparent. For example, when a comet is at a great 
distance without the earth’s orbit its tail is foreshortened ; but, 
was attributed, in part, the great brilliancy of the comet 
before its orbit had ‘boon dibekalthiod But the variations in the 
fac if not universal. The facts noticed are that the nebu- 
ity of the head contracts greatly as the comet approaches the 
sun, and enlarges, in a corresponding degree, as it recedes from 
the sun. These curious and puzzling facts are but simple con- 
sequences of the dynamical theory that has now been developed. 
On referring to equ. (8) it will be seen that the repulsive force 
of the ntcleus exceeds that of the sun, at the distance of the 
nucleus, in the ratio of the focal distance NV (fig. 1) to the ra- 
dius of the nucleus. Unless, therefore the repulsion of the nu- 
€us varies with its distance from the sun, the ratio of these 
forces must vary inversely as the square of the distance from 
po "@) The focal distance and all the distances Z, given by 
a. should therefore 
to the same law, as the comet approaches the sun, and recedes 
ing to 
- easurements of G. P. Bond, Esq., of Harvard College Ob- 
Servatory, more rapid than showid b 
SECOND SERIES, Vos. XXVII, No. 70.—JAN., 1869 
13 : 
te 
