A typical _spiral_ nebula is composed of a disc-shaped central portion, with long curved arms projecting from opposite sides of it, which give an impression of rapid rotatory movement. The discovery of spiral nebulæ was made by Lord Rosse with his great 6-foot reflector. Two good examples of these objects will be found in Ursa Major, while there is another fine one in Canes Venatici (see Plate XXII., p. 314), a constellation which lies between Ursa Major and Boötes. But the finest spiral of all, perhaps the most remarkable nebula known to us, is the Great Nebula in the constellation of Andromeda, (see Plate XXIII., p. 316)--a constellation just further from the pole than Cassiopeia. When the moon is absent and the night clear this nebula can be easily seen with the naked eye as a small patch of hazy light. It is referred to by Al Sufi. [Illustration: PLATE XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES VENATICI From a photograph by the late Dr. W.E. Wilson, D.Sc., F.R.S. (Page 314)] Spiral nebulæ are white in colour, whereas the other kinds of nebula have a greenish tinge. They are also by far the most numerous; and the late Professor Keeler, who considered this the normal type of nebula, estimated that there were at least 120,000 of such spirals within the reach of the Crossley reflector of the Lick Observatory. Professor Perrine has indeed lately raised this estimate to half a million, and thinks that with more sensitive photographic plates and longer exposures the number of spirals would exceed a million. The majority of these objects are very small, and appear to be distributed over the sky in a fairly uniform manner. _Planetary_ nebulæ are small faint roundish objects which, when seen in the telescope, recall the appearance of a planet, hence their name. One of these nebulæ, known astronomically as G.C. 4373, has recently been found to be rushing through space towards the earth at a rate of between thirty and forty miles per second. It seems strange, indeed, that any gaseous mass should move at such a speed! What are known as _ring_ nebulæ were until recently believed to form a special class. These objects have the appearance of mere rings of nebulous matter. Much doubt has, however, been thrown upon their being rings at all; and the best authorities regard them merely as spiral nebulæ, of which we happen to get a foreshortened view. Very few examples are known, the most famous being one in the constellation of Lyra, usually known as the Annular Nebula in Lyra. This object is so remote from us as to be entirely invisible to the naked eye. It contains a star of the fifteenth magnitude near to its centre. From photographs taken with the Crossley reflector, Professor Schaeberle finds in this nebula evidences of spiral structure. It may here be mentioned that the Great Nebula in Andromeda, which has now turned out to be a spiral, had in earlier photographs the appearance of a ring.