64 Observations of a Comet. 



irregular round, very gradually much brighter in the mid- 

 dle." There is a very faint diffused nebulosity on the north- 

 preceding side; I take it to be the vanishing remains of the 

 comet's tail. 



Feb. 19. Considerably bright; about |th of the field = 

 &' 26' ** in diameter, gradually brighter in the middle. " 

 The faint nebulosity in the place where the tail used to be, 

 still projects a little further from the centre than in other 

 directions. 



1 Feb. 21 . Less bright than on the 19th : nearly of the same 

 size : gradually brighter in the middle. The nebulosity still 

 a little projecting on the side where the tail used to be. 



Result of the foregoing Observations, 

 From the observations which are now before us, we may 

 draw some inferences, which will be of considerable im- 

 portance with regard to the information they give us, not 

 only of the size of the comet, but also of the nature of its 

 illumination. 



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 

 experimentally 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, <5h. 20', was not quite so large as that of 

 the third satellite of Jupiter. In order therefore to have 

 some idea of the real magnitude of our comet, we may ad- 

 mit that its diameter at the time of observation was about 

 1", which certainly cannot be far from truth. The diameter 

 of the third 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 di- 

 stance from the Earth at which, in my observation, it was 

 compared with the diameter of the comet, and thus more 



accuracy, 



