297 



tion between the stomach and spleen is carried on; but the evidence 

 is too strong to leave a doubt on the mind of the author as to their 

 existence. 



Observations of a Comet, made with a View to investigate its Magnitude 

 and the Nature of its Illumination. To which is added, an Account 

 of a new Irregularity lately perceived in the apparent Figure of the 

 Planet Saturn. By William Herschel, LL.D. F.R.S. Read 

 April 7, 1808. [Phil. Trans. 1808,^. 145.] 



Dr. Herschel presuming that the motion of the comet would be 

 correctly ascertained at the Royal Observatory at Greenwich, confined 

 his observations to its physical condition, and relates the several cir- 

 cumstances which he remarked concerning its nucleus, its head, its 

 coma, and its tail. The form of its nucleus was particularly attended 

 to on the 4th of October, and remarked to be perfectly circular and 

 equally bright on all sides. 



Its magnitude was at the same time estimated at about 3", due 

 care being taken to determine that the visible disc was not spurious, 

 by means described in a former paper. It was examined with a 10- 

 feet reflector, and with powers of 200 and 300 ; but its light was not 

 found sufficient to bear higher powers. 



On the 19th at 5" 40' it was again estimated at 2f ", but at 6 h 20' 

 it was compared with Jupiter's third satellite, and found to be rather 

 less than it. 



The coma or nebulous appearance surrounding the head was at the 

 same time estimated at about six minutes, and on the 6th of Decem- 

 ber at 4" 45'. 



The tail on the 18th of October measured 3|, but on the 6th of 

 December it was reduced to 23'. The tail was at various times ob- 

 served to appear longer and more distinct on the south preceding, 

 than on the north following, side. 



Various authors having said that the tails of comets were so rare 

 as not to affect the light of the smallest stars seen through them, Dr. 

 Herschel paid particular attention to that circumstance ; but uni- 

 formly found those stars which he saw emerge from behind the tail 

 to become brighter than before in comparison to neighbouring stars. 



On the 6th of December the head had the appearance of a large 

 brilliant round nebula, suddenly much brighter in the centre. On the 

 1st and 14th of January it had still more the appearance of a nebula, 

 with an increase of brightness in the middle ; but even so late as the 

 21st of February, the nebulosity was observed to project a little on 

 that side where the tail used to be seen. 



The inferences which Dr. Herschel draws from these observations, 

 are, first, that the body of the comet consists of solid matter about 

 538 miles in diameter ; secondly, that since the portion of its visible 

 hemisphere which could be illuminated by the sun on the 4th of Oc- 

 tober did not exceed 120, and since it did not appear gibbous but 

 perfectly circular, the surface of the comet must itself be luminous, 



