The Iridescent Clouds

Abstract

THE coloured fringes and bows described by Mr. N. in Prof. C. Piazzi Smyth's communication (p. 316) are clearly of a totally different character from the iridescent clouds that were so widely remarked in December. I take the “fringes and bows in circles” mentioned by him to be simply the same phenomenon of coloured circles that is so often seen around the moon, which goes by the name of a “corona”; and the reason why it is not easily seen around the sun, except by reflection in glass or water, is that the sun is too dazzling to look at directly. There is another phenomenon of coloured clouds which is probably also alluded to by Mr. N., and that is when thin clouds, usually cirrus, show interference colours, often very vividly; the positions of these colours evidently depending on the structure of the clouds, and being quite irregular with reference to the sun. The iridescent clouds recently observed no doubt owe their colour to the same cause, but the kind of cloud was evidently different, and the colours produced were much more striking. The clouds themselves were quite recognisable as being of a peculiar type, even when too far from the sun to show any colour. The clouds thus coloured are usually of a much striated or rippled structure, and show the colours generally in small spectra; whereas the clouds seen in December were remarkably smooth in texture, and although often striated, the striations were feeble and comparatively few, and in straight lines, while each cloud showed one regular gradation of colour.