made with a viezv to investigate its Magnitude Sec. 155 
the size of the comet, but also of the nature of its illumina- 
tion. 
A visible, round and well defined disk, shining in every 
part of it with equal brightness, elucidates two material cir- 
cumstances ; for since the nucleus of this comet, like the body 
of a planet, appeared in the shape of a disk, which was ex- 
perimentally found to be a real one, we have good reason to 
believe that it consists of some condensed or solid body, the 
magnitude of which may be ascertained by calculation. For 
instance, we have seen that its apparent diameter, the 19th of 
October, at 6 h 20', was not quite so large as that of the 3d 
satellite of Jupiter. In order therefore to have some idea of 
the real magnitude of our comet, we may admit that its 
diameter at the time of observation was about 1", which cer- 
tainly cannot be far from truth. The diameter of the 3d 
satellite of Jupiter, however, is known to have a permanent 
disk, such as may at any convenient time be measured with 
all the accuracy that can be used ; and when the result of such 
a measure has given us the diameter of this satellite, it may 
by calculation be brought to the distance from the earth at 
which, in my observation, it was compared with the diameter 
of the comet, and thus more accuracy, if it should be required, 
may be obtained. The following result of my calculation 
however appears to me quite sufficient for the purpose of a 
general information. From the perihelion distance 0,64,7491, 
and the rest of the given elements of the comet, we find that 
its distance from the ascending node on its orbit at the time 
of observation was 73 0 45' 44" ; and having also the earth’s 
distance from the same node, and the inclination of the comet’s 
orbit, we compute by these data the angle at the sun. Then 
X 2 
