* 267 j Brightness of Stars : Planetary Observations 349 



himself to have seen also (1798) four other satellites of 

 Uranus, but their existence was never satisfactorily verified ; 

 and the second pair of satellites now known to belong to 

 Uranus, which were discovered by Lassell in 1847 (chap- 

 ter xiii., 295), do not agree in position and motion with 

 any of Herschel's four. It is therefore highly probable that 

 they were mere optical illusions due to defects of his mirror, 

 though it is not impossible that he may have caught glimpses 

 of one or other of Lassell's satellites and misinterpreted the 

 observations. 



Saturn was a favourite object of study with Herschel from 

 the very beginning of his astronomical career, and seven 

 papers on the subject were published by him between 1790 

 and 1806. He noticed and measured the deviation of the 

 planet's form from a sphere (1790); he observed various 

 markings on the surface of the planet itself, and seems to 

 have seen the inner ring, now known from its appearance 

 as the crape ring (chapter XIIL, 295), though he aid not 

 recognise its nature. By observations of some markings at 

 some distance from the equator he discovered (1790) that 

 Saturn rotated on an axis, and fixed the period of rotation 

 at about loh. i6m. (a period differing only by about 2 

 minutes from modern estimates), and by similar observations 

 of the ring (1790) concluded that it rotated in about io| 

 hours, the axis of rotation being in each case perpendicular 

 to the plane of the ring. The satellite Japetus, discovered 

 by Cassini in 1671 (chapter vin., 160), had long been 

 recognised as variable in brightness, the light emitted being 

 several times as much at one time as at another. Herschel 

 found that these variations were not only perfectly regular, 

 but recurred at an interval equal to that of the satellite's 

 period of rotation round its primary (1792), a conclusion 

 which Cassini had thought of but rejected as inconsistent 

 with his observations. This peculiarity was obviously capable 

 of being explained by supposing that different portions of 

 Japetus had unequal power of reflecting light, and that like our 

 moon 't turned on its axis once in every revolution, in such 

 a way as always to present the same face towards its 

 primary, and in consequence each face in turn to an 

 observer on the earth. It was natural to conjecture that 

 such an arrangement was general among satellites, and 



