These crowns may be observed generally by night round the moon and other stars, but very seldom by day; in fact, so rarely in the latter case, that certain of the Greeks have denied that they appear at all by day. But history proves that they do. The cause of the infrequency of their appearance by day is that the sun’s light is stronger then, and the atmosphere itself when stirred and warmed by it is less dense. The moon’s power, on the other hand, is feebler, and is therefore more easily resisted by the surrounding air. The rest of the heavenly 11 bodies are equally weak, and unable by their own force to burst through the atmosphere. So their shape is impressed and retained in the more solid and less yielding medium. For, in order to produce the phenomenon, the atmosphere must neither be so thick as to exclude or dissipate the light that streams in on it, nor yet so thin and rare as to furnish no hold to the rays that fall upon it. This particular consistency is obtained at night: the sluggish air is at that time struck with the faint light from moon or stars without violence or rudeness, and, being thicker than it is wont to be by day, is tinged thereby. III On the contrary, _the Rainbow_ does not occur by night, except on 1 very rare occasions, inasmuch as the moon has not sufficient strength to pierce the clouds and suffuse them with hues such as they receive from the brilliant light of the sun. The shape and varied colours of the rainbow are due to the peculiarities of different kinds of clouds. Some parts of the clouds are swollen, others hollow; some are too dense to transmit sunlight, others too rare to exclude it. This difference in 2 consistency causes alternations of light and shade, and produces that marvellous variety presented by the rainbow. Another explanation is offered in instances like the following: When a pipe bursts anywhere, the water is observed to be forced by pressure through the small opening; the drops seen against a slanting sun reproduce the appearance of the rainbow. Again, if you will at any time watch a fuller at work, you will observe the same appearance: when he has filled his mouth with water and spirts it lightly on the clothes stretched on pegs, the air thus besprinkled exhibits plainly the various colours that shine in the bow. One cannot doubt that the reason of this lies in the moisture. 3 For a rainbow never occurs except when there are clouds about.