Saturn has perhaps received more than its share of attention owing to these rings. This led to other discoveries. Huyghens in 1655, and J. D. Cassini in 1671, discovered the sixth and eighth satellites (Titan and Japetus). Cassini lost his satellite, and in searching for it found Rhea (the fifth) in 1672, besides his old friend, whom he lost again. He added the third and fourth in 1684 (Tethys and Dione). The first and second (Mimas and Encelades) were added by Herschel in 1789, and the seventh (Hyperion) simultaneously by Lassel and Bond in 1848. The ninth (Phoebe) was found on photographs, by Pickering in 1898, with retrograde motion; and he has lately added a tenth.
The occasional disappearance of Cassini's Japetus was found on investigation to be due to the same causes as that of Jupiter's fourth satellite, and proves that it always turns the same face to the planet.
_Uranus and Neptune_.--The splendid discoveries of Uranus and two satellites by Sir William Herschel in 1787, and of Neptune by Adams and Le Verrier in 1846, have been already described. Lassel added two more satellites to Uranus in 1851, and found Neptune's satellite in 1846. All of the satellites of Uranus have retrograde motion, and their orbits are inclined about 80° to the ecliptic.
The spectroscope has shown the existence of an absorbing atmosphere on Jupiter and Saturn, and there are suspicions that they partake something of the character of the sun, and emit some light besides reflecting solar light. On both planets some absorption lines seem to agree with the aqueous vapour lines of our own atmosphere; while one, which is a strong band in the red common to both planets, seems to agree with a line in the spectrum of some reddish stars.
Uranus and Neptune are difficult to observe spectroscopically, but appear to have peculiar spectra agreeing together. Sometimes Uranus shows Frauenhofer lines, indicating reflected solar light. But generally these are not seen, and six broad bands of absorption appear. One is the F. of hydrogen; another is the red-star line of Jupiter and Saturn. Neptune is a very difficult object for the spectroscope.
Quite lately [7] P. Lowell has announced that V. M. Slipher, at Flagstaff Observatory, succeeded in 1907 in rendering some plates sensitive far into the red. A reproduction is given of photographed spectra of the four outermost planets, showing (1) a great number of new lines and bands; (2) intensification of hydrogen F. and C. lines; (3) a steady increase of effects (1) and (2) as we pass from Jupiter and Saturn to Uranus, and a still greater increase in Neptune.