PART OF A GREAT HALO.] [Illustration: Plate 2. PART OF A SOLAR HALO.] These circles, and the bright spots called mock-suns or mock-moons which often accompany them, can all be explained on the assumption that their cause is the passage of light through a veil composed of hexagonal crystals of ice. The simple halo of 22 degrees radius is common in most parts of the world, being very generally formed by the film of high cloud which marks the advancing edge of a cyclonic cloud system. A portion of one is shown in Plate 2, in which the rudimentary fibrous structure of the sheet of cloud is distinctly seen. Halos of this sort are frequently coloured, often most brilliantly so; but the tints are seldom noticed unless a black mirror is used. They are sometimes quite as bright as those of an ordinary rainbow, but instead of being projected upon a background of dark rain-clouds, they are seen against a part of the sky which is near the sun, and therefore exceptionally bright. The red is always on the inside of the ring, the violet outside, thereby distinguishing them at once from the so-called coronæ, which are formed around the sun or moon when shining through a sheet of alto or other lower cloud made up of liquid particles. In these the radius of the rings is much less, and the red is on the outside, the violet actually touching the central luminary. The cloud which produces halos is called cirro-nebula. It is much thinner, and on an average higher than cirro-stratus. Mr. Ley named it cirro-velum (or cirro-veil), but cirro-nebula has now got to be fairly well understood. It sometimes appears and disappears in a curious manner, showing that it occurs in patches, which drift about or which keep forming and melting away, only to repeat the process. If, however, it forms part of an advancing cyclone fringe, then the sky gets whiter and whiter, until it is covered with a sheet of undoubted cirro-stratus. This process of growing density is shown in progress in Plate 3. [Illustration: Plate 3. CIRRO-NEBULA CHANGING TO CIRRO-STRATUS.] Cirro-nebula, as we shall call it, floats at very great altitudes in temperate regions; but in polar latitudes, where the optical phenomena peculiar to it are most brilliant and diversified, it seems probable that the ice dust is much lower down, even in actual proximity to the ground. In England its height varies greatly with the time of year, and other circumstances, but mounts up in summer to such altitudes as nine miles or more; the greatest height yet recorded being 9·6 miles, or about 15,500 metres, at Exeter.