APPENDIX. 11 



It is evident that Herschel's II. and IV. are Lassell's Titania 

 and Oberon. 



Lassell's station at Malta was much better in regard to clear- 

 ness of sky than Herschel's in England, his instrumental means 

 were far superior, and the altitude of Uranus was greater at Malta 

 in 1864 than in England in n98, so that we must assume that if 

 Lassell could not see Herschel's I., III., Y., and VI., they did not 

 exist. 



In his report of his observations (Memoirs R. A. S., vol. 36) 

 Lassell says that he repeatedly scrutinized the vicinity of the 

 planet for the purpose of detecting faint satellites exterior to 

 Oberon, and that he never suspected the existence of any such. 

 Therefore, in the examination of Herschel's observations, I shall 

 reject all those referring to satellites V. and VI., and for the 

 same reason I shall reject all those referring to satellite III. ; in 

 this latter case we have the added testimony of five months' ob- 

 servations with the Alvan Clark refractor of the U. S. Naval 

 Observatory at Washington. 



There remain, then, of Herschel's observations only those of 

 suspected interior satellites which it will be profitable to examine. 



Before selecting any of these observations for discussion it is 

 necessary to premise a few words in regard to Herschel's method 

 of observation. On a very few occasions he was able to faintly 

 illuminate the wires of his micrometer for a determination of the 

 position of Oberon and Titania, but all of his estimations of the 

 position of any saiall objects interior to these had to be made in 

 a perfectly dark field. Hence these estimations are liable to a 

 large error of from 5 to 15 degrees in position angle. Owing to 

 the glare of the planet in the field of the telescope Herschel found 

 that he could seldom see Oberon nearer to the planet than 23.6", 

 while Titania was usually invisible at distances less than 18.1". 

 Of course, under ordinary circumstances, Ariel and LTmbriel could 

 not be visible at all, but there were occasions when the fine polish 

 of his mirror or the good state of the atmosphere permitted him 

 to view objects even as close as 10". It was evidently impossi- 

 ble for him to see an interior satellite on two consecutive nights, 

 and of this he was fully aware. 



It was his habit to make his observations of the " first" and 

 "second" satellites {i. e. of Titania and Oberon), and to map 

 down all small stars near to the planet. On the next subsequent 

 observing night he examined the spot where the planet had been, 

 and was thus able to identify all small stars as such. In his 

 printed observations the sketches of star configurations are not 

 given, but his remarks in full are quoted, followed by an " identi- 

 fication," as he calls it, of all suspected satellites. The patience 

 and skill with which these identifications are carried out year 

 after year are truly admirable, and they give a real value to that 



(31) 



