Rainbows on Scum


I HAVE several times noticed “rainbows” on the black scum upon the pond in a park in this town, and have imagined all of them to have been formed on dew deposited upon a film of soot. It is out of the question, however, that yesterday there can have been any dew to produce the phenomenon, as there had been a thick hoar frost on the grass, all melted by the warm sun by 10.30 a.m., at which time there was a very vivid double “rainbow,” which seemed exactly like an ordinary bow upon rain, except that there were none of the supernumerary arcs due to diffraction, and that the outer bow was fainter than usual in proportion to the inner one. The pond was thinly frozen over, but, it being a cloudless day, the surface of the ice was by that time covered with water. On closely examining the scum, I found it was composed of floating black particles, I presume of soot (the weather being rather foggy), and to many of these, minute drops were adhering, which varied much in size, the largest being probably 1/50 of an inch in diameter. It was surprising to find distinct drops upon water, but I suppose it must have been the particles of soot that kept them separate. It seems probable they were a portion of the melted hoar frost; but it is rather curious that in such a situation this can produce a rainbow, seeing that usually melted hoar frost does not do so at all, or at most gives a very slight one; so decidedly is this the case, that one may distinguish in the morning between dew and melted hoar frost by noticing whether a “rainbow” and white anthelion are formed; dew being capable of producing bright ones owing to the roundness of the drops composing it, while hoar frost when it melts usually turns into irregular drops. I may say, however, that this “rainbow” on the pond was far more brilliant than any ordinary dew bow, and therefore it would appear that there is some property in the particles of soot to perfect the roundness of drops adhering to them, and so produce a striking phenomenon even from melted hoar frost.

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BACKHOUSE, T. Rainbows on Scum. Nature 43, 416 (1891) doi:10.1038/043416a0

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