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Art. LXII.—Observations on Rainbows.
[Read before the Wellington Philosophical Society, 7th December, 1892.]
So many conditions are necessary for the appearance of a perfect rainbow that the sight of one is really very rare. The sun must be shining clearly in one quarter of the heavens, and in the opposite quarter there must be an unbroken rain-cloud screen of sufficient extent for the projection of the bow. In nearly all cases one or both bows are only fragmentary. I have not the knowledge of optics necessary to deal with the subject in a scientific manner, and only wish to note some unusual phenomena in connection with rainbows, of a kind not provided for in popular text-books. I should like to know if others have observed similar appearances. That they may, if they watch for them, I do not doubt. Usually one sees no more than one looks for or expects to see, unless for some reason special attention is given to the object observed.
The ordinary primary and secondary rainbow of the textbooks are familiar to us all. But the first point I would note is that the closest observation often fails to detect any sign of the secondary rainbow, even when all the conditions are apparently favourable for its appearance.
About three years ago, in Napier, looking at an unusually brilliant rainbow, I was struck with the great breadth of the band of colour. Looking more closely, I saw that the spectrum was double. This was also the case with the portion of the secondary rainbow which was visible at the time; but in this case the additional bands were so faint that I should not have seen them had I not been led to look for them through having noticed them in the primary bow.
Shortly afterwards, looking at another bow, I was unable to discern the second series of colours either in the primary or secondary; but, to my surprise, I distinctly saw a tertiary bow, the colours, as might have been expected, following the order of the primary.
Both of these phenomena—the double series of colours in one bow, and the tertiary bow—I noticed on other occasions
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in Napier during the same rainy season, but on no occasion did I see both at the same time.
In Wellington, last June, seeing a fine bow, I looked to see if it presented any peculiar feature. It was only on close examination that I detected that this one was double; and, to my surprise, I saw what I had not noticed before, that in the second spectrum, forming the inner portion of the bow, the colours followed the same order as the outer and brighter series. I failed to see any colourless space between to represent the invisible portions of the spectra, and thence inferred that they must have to some extent overlapped.
I do not attempt to account for these variations in the appearance of the bow, but should now scarcely be surprised, under more than usually favourable conditions for observation, to see at the same time three double rainbows in the heavens.
Postscript.—Mr. T. B. Harding, to whom a copy of the above note was sent, writes,—
“On reading your paper, I could not account for the doubling of the spectrum; but I have since met with a scientific note which I think may afford an explanation of the cause. It appears that often the larger drops of rain are not solid, but hollow spheres, as they are found to mark what they fall on in rings [○], while the others make spots [•]. Now, it seems evident that, as a glass globe empty, or, rather, filled with air, will refract differently from one filled with water, so the two classes of raindrops will give differing spectra (in position), yet, as we may imagine, so close together as to appear continuous.
“With regard to the third rainbow, I do not see how it could be formed in the ordinary way, as it would need three reflections within the sphere, and two refractions, one on entering and one on leaving. But there is another way in which they may be formed—as a diffraction spectrum. I saw one of them on Friday afternoon, the 11th November. The sky where it was was clear. The arch was about 60°, and resting at the ends each on a cloud. Turning towards the sun, it appeared covered with a granulous cloud which acted as a kind of grating or diffracting medium, and the effect was perfect. It was not raining where I was, and did not appear to be where the bow was seen.
“I do not expect that you will ever see three double rainbows at the same time.”