Prof. Forbes has again attacked the question of a possible ultra-Neptunian planet, and from a consideration of the comets of 1556, 1843 I, 1880 I, and 1882 II, finds a mean distance of 105·4, with an inclination of the orbit of 52° to the plane of the ecliptic. This high inclination implies that "during the greatest part of its revolution it is beyond the zodiac," and this, Mr. W. T. Lynn thinks, "may partly account for its not having hitherto been found by observation."[181] From a consideration of the approximately circular shape of the orbits of all the large planets of the solar system, Dr. See suggests the existence of three planets outside Neptune, with approximate distances from the sun of 42, 56, and 72 respectively (earth's distance = 1), and recommends a photographic search for them. He says, "To suppose the planetary system to terminate with an orbit so round as that of Neptune is as absurd as to suppose that Jupiter's system terminates with the orbit of the fourth satellite."[182] According to Grant, even twenty years before the discovery of Neptune the error of Prof. Adams' first approximation amounted to little more than 10°.[183] CHAPTER XI Comets We learn from Pliny that comets were classified in ancient times, according to their peculiar forms, into twelve classes, of which the principal were: _Pogonias_, bearded; _Lampadias_, torch-like; _Xiphias_, sword-like; _Pitheus_, tun-like; _Acontias_, javelin-like; _Ceratias_, horn-like; _Disceus_, quoit-like; and _Hippias_, horse-mane-like.[184] Of the numerous comets mentioned in astronomical records, comparatively few have been visible to the naked eye. Before the invention of the telescope (1610) only those which were so visible _could_, of course, be recorded. These number about 400. Of the 400 observed since then, some 70 or 80 only have been visible by unaided vision; and most of these now recorded could never have been seen without a telescope. During the last century, out of 300 comets discovered, only 13 were very visible to the naked eye. Hence, when we read in the newspapers that a comet has been discovered the chances are greatly against it becoming visible to the naked eye.[185] Although comparatively few comets can be seen without a telescope, they are sometimes bright enough to be visible in daylight! Such were those of B.C. 43, A.D. 1106, 1402, 1532, 1577, 1744, 1843, and the "great September comet" of 1882.