Again, a marked contrast is shown in Plate 23. Here we have the highest and thinnest form of cirro-cumulus, the one named cirro-macula by Mr. Ley. It is rarely, if ever, seen before eleven o’clock in the morning, and is far commoner in the afternoon. The example shown was photographed at sunset at the close of a day which had been almost cloudless. Cirro-macula forms here and there in a clear sky. A hazy, whitish patch appears, which at first shows no definite structure, but looks almost like a little bit of cirro-nebula. This suddenly splits up by clear blue lanes running through it, and cutting the patch up into irregular segments, which quickly round themselves off into minute bits usually whiter on their edges and semi-transparent in the centre. The process can be strikingly imitated by scattering on water some fine powder which will float. If left without disturbance, the particles draw together into numerous small groups, leaving lanes of clear water between them. [Illustration: Plate 23. SPECKLE CLOUD (Ley). (_Cirro-macula._)] Cirro-macula frequently gives rise to the fibrous form of cirrus we have called cirrus caudatus. The granules of the cirro-macula grow denser, and begin to drop their frozen particles as soon as they become large enough. Indeed, a cloudlet of cirro-macula may sometimes be seen to turn bodily into a fine line of falling crystals, which will be a curving line of cirrus. On the other hand, it will sometimes remain visible for an hour or more without any trace of descending streaks or floating fibres. Pure cirro-macula such as Plate 23 is not often seen; it is far more frequently mixed with more solid-looking cloudlets and descending fibres, such as are shown in Plate 24, which gives the same point of view as 23, but a quarter of an hour later, and photographed with a longer focus lens. These two photographs, together with 20 and 21, give excellent examples of the use of the black mirror. In none of the four could the naked eye detect all of the cloud structures. The whole sky was a blaze of dazzling light, but by adjusting exposure and development the details are fully brought out without the least difficulty. [Illustration: Plate 24. CIRRUS CAUDATUS AND CIRRO-MACULA.] Cirro-stratus, we see from the examples which have been considered, hardly deserves to be treated as a distinct genus of cloud. Its formation is identical with that of many species of cirrus, or in some cases with that of the speckle cloud, cirro-macula, or even the coarser kinds of cirro-cumulus. The different varieties which it shows are best rendered by reference to the specific names of the detached forms which have similar features.