One more kind of nebula, the most numerous of all, remains to be mentioned, the so-called “white nebulae,” which do not glow green like many of the brighter planetaries, but shine with a white light and have more or less star-like spectra, although not even the most powerful telescopes can resolve the white cloudiness into stars. The typical nebula of this class is the famous Andromeda Nebula, visible to the naked eye in northern skies as a large oval spot shining softly “like a candle shining through horn.” Photography first disclosed the remarkable fact that it has the form of a great, closely-wound spiral, and further research has shown that by far the greater number of “white nebulae” have this form. There is a very fine one in Aquarius,[12] which has been known since 1824, but visual observations gave absolutely no idea of its true form. A photograph exposed for four hours in September 1912 showed it clearly as about two turns of a great spiral. [12] N. G. C. 7293. The distribution of this kind of nebula is quite different from that of the gaseous nebulae, for, instead of clustering towards the Milky Way, they avoid it, and especially the brightest region, where we saw that the others most abound, viz. in Scorpio, Sagittarius, and Ophiuchus. On the contrary, the largest number of these is found near the north pole of the Galaxy—that is, as far removed from it as possible, in Virgo. There is, however, no corresponding group about the south pole of the Galaxy. One investigator has found the distance of the Andromeda Nebula to be twenty light-years, but the distance and the movements of this type are difficult to discover. They are evidently very different from the others, and quite as mysterious. XVI THE CLOUDS OF MAGELLAN One of the wonders which most attracted the attention of early explorers in the southern hemisphere, and roused as much interest as the Southern Cross, was the pair of faint clouds, looking like detached pieces of the Milky Way, which are seen in the neighbourhood of the South Pole. Marco Polo made a sketch of the Greater Cloud, which he describes wonderingly as “a star as big as a sack.” Although some star-maps show short branches of Milky Way pointing towards the two Clouds, this is incorrect, and they are quite separate from it. Herschel was struck by their isolation, especially in the case of the Little Cloud, which he described as situated in a “most oppressively desolate desert,” its only neighbour being the globular cluster 47 Toucani, which is near, but separated by a perfectly black sky. The Greater Cloud is much brighter to the naked eye than the Lesser, and it is much more complex and interesting in the telescope. It contains, moreover, the wonderful Looped Nebula, of which we have already spoken.