The change from cirro-nebula to cirro-stratus is generally accompanied by the formation of a distinct fibrous structure, easily observable by the black mirror. This is not really a new feature, but only a further development of a structure already existing, but too minute to be easily seen. True halo-producing cirro-nebula may usually be shown to possess more or less of a fibrous texture in an indirect way, which is worth a brief description. In order to observe the spots on the sun and other features of the solar surface, it is a common practice to hold a white screen, say, about a foot from the eyepiece of a telescope, while the instrument is pointed to the sun. An image, considerably magnified, is thus projected on to the screen, and the solar details can be studied with ease and safety. If thin clouds drift before the sun, their images are similarly projected as they pass across its disc, and it is possible thus to detect not only the fibrous texture but also the movement of cirro-nebula.[2] The change into cirro-stratus is also attended by a marked fall in altitude, but whether this is due to an actual descent of the cloud particles, or to a downward spread of the conditions which give rise to them, cannot at present be definitely settled. The balance of probability points very strongly towards the downward spread of the conditions. It is likely that the clouds, particularly the cyclonic specimens, are wedge-shaped, and that as they pass overhead we see first the thin edge, and later on the thicker parts, which project much lower down. This is just one of those many minor problems in cloud mechanics which we are not able to solve from the scanty data on record. Occasionally cirro-nebula breaks up into little detached semi-transparent cloudlets, all of them exceedingly thin, and showing a complicated mottling, resembling, on a minute scale, the ripple clouds of much lower altitudes. Such a sky is depicted in Plate 4, but no reproduction can possibly do justice to the minute and delicate features of the real thing. The arrangement of the faint markings was in a state of continual flux, curiously similar to the ever-changing aspect of the sun’s photosphere when seen under adequate power. Some parts of the cloud stratum would at one moment break up into distinct granules arranged in complicated patterns, other parts would assume a fibrous texture, and yet other places would show a continuous smooth sheet. In a minute or two all would be changed--the smooth part granulated, the fibres vanished, and the granules fused together, and so on, no two of a series of photographs representing the same details. [Illustration: Plate 4. CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.]