Note 36 — Chapter VIII

The phenomenon here depicted is closely allied to those tinted halos which are seen in so much diversity and under so many modifications around both sun and moon. In Schumacher’s ‘Astronomische Jahrbücher’ (Altona, 1823) Fraunhofer has detailed the theory of these halos, and has proved his assertions by many examples that had fallen under his own observation. Whenever the sun or the moon is surrounded by a halo, the sky is ordinarily veiled in light vapours. If the phenomenon is perfect, the rings of this halo are seen to be of the colours of the rainbow. Fraunhofer divides these halos into two classes: viz., halos of a small and halos of a large diameter. If the red tint is outside and away from the luminous body, as in the present case, he calls it a halo of the smaller kind; but if the red is inside and next to the luminous body, it is a halo of the larger kind. This latter case is closely allied to the phenomenon of parhelia. The cause of these tinted halos is to be found in a diffraction of light through globules of vapour, and Fraunhofer has given proof that the light, in passing across the edges of these globules, would assume an appearance of diffraction similar to that which would be caused by its passing through minute apertures. For the formation of a tinted circle it is necessary that the globules should be equally diffused and of an equal magnitude. If the globules were very irregular, there would be only a bright glare, because the eye would receive rays of various colours from one and the same spot in the atmosphere; then the result would be that the light would be white, as in the case under our notice it appeared directly round the outline of the cloud, and also beyond the outside ring of red, so that the coloured circle was bounded on each side by a rim of white light. The smaller the globules of vapour, the larger are the tinted rings, for according to the theory the diameters of the rings are in inverse ratio to those of the globules. According to another theory represented by Galle (Poggendorf’s ‘Annalen,’ vol. xlix.), one cause of these tinted halos is the presence in the atmosphere of ice-crystals of microscopic minuteness; but this hypothesis seems confuted by the fact that similar phenomena have frequently been witnessed within the tropics (Alex. von Humboldt, Voyage II., p. 309). This phenomena of the 18th of May, 1869, was remarkable for the form of the tinted circle, which corresponded exactly with the accidental outline of the clouds, which presented a threefold curve, thus . Thus the entire rim of the cloud became a series of luminous sunlight points formed of globules of vapour, making a halo of the smaller class, and sending forth their own shadows.