214 PHILOSOPHICAL TRANSACTIONS. [aNNO l/SJ. 



thing resembling tremella, or that kind of green matter, or water moss, which 

 forms on the bottom and sides of the vessel when this water is suffered to remain 

 in it for a considerable time, and into which Dr. Ingenhousz supposes the ani- 

 malcules above-mentioned to be actually transformed. 



XFl. Discovery of Two Satellites revolving round the Georgian Planet. By 

 Wm, Herschel, LL.D.y F.R.S. p. 125. 



In the beginning of last month (Jan. 1787) I was often surprised when I 

 reviewed nebulae that had been seen in former sweeps, to find how much brighter 

 they appeared, and with how much greater facility I saw them. The cause of 

 it could be no other than the quantity of light that was gained by laying aside 

 the small speculum, and introducing the front view. It would not have been 

 pardonable to neglect such an advantage, when there was a particular object in 

 view, where an accession of light was of the utmost consequence. The 11th 

 of January therefore, in the course of my general review of the heavens, I 

 selected a sweep which led to the Georgian planet; and while it passed the meri- 

 dian I perceived near its disc, and within a few of its diameters, some very faint 

 stars whose places I noted down with great care. 



The next day, when the planet returned to the meridian, I looked with a 

 most scrutinizing eye for my small stars, and perceived that two of them were 

 missing. Had I been less acquainted with optical deceptions, I should immedi- 

 ately have announced the existence of one or more satellites to our new planet; 

 but it was necessary that I should have no doubts. The least haziness, other- 

 wise imperceptible, may often obscure small stars; and I judged therefore that 

 nothing less than a series of observations ought to satisfy me, in a case of this 

 importance. To this end I noticed all the small stars that were near the planet 

 the 14th, 17th, 18th, and 24th of January, and the 4th and 5th of February; 

 and though at the end of this time I had no longer any doubt of the existence 

 of at least one satellite, I thought it right to defer this communication till I 

 could have an opportunity of seeing it actually in motion. Accordingly I began 

 to pursue this satellite on Feb. the 7th, about 6 o'clock in the evening, and kept 

 it in view till 3 in the morning oiji Feb. the 8th; at which time, on account of 

 the situation of my house, which Intercepts a view of part of the ecliptic, I was 

 obliged to give over the chace: and during those Q hours I saw this satellite faith- 

 fully attend its primary planet, and at the same time keep on, in its own course, 

 by describing a considerable arch of its proper orbit. 



While chiefly attending to the motion of this satellite, I did not forget to 

 follow another small star, which I was pretty well assured was also a satellite, 

 especially as I had, on the night of the 14th of January, observed 2 small stars 

 which were wanting on the 17th, and again missed other 2 the 24th which had- 



