After the Deluge
- The Rainbow Bridge: Rainbows in Art, Myth and Science by Raymond Lee and Alistair Fraser
Pennsylvania State, 394 pp, £54.95, June 2001, ISBN 0 271 01977 8
First the rainbow brought messages, later it demanded explanations. In the story of Noah it is God’s promise of an end to floods; in Greek mythology, Iris was both goddess of the rainbow and the messenger of the gods. Then, once a scientific theory was called for, it proved far from easy to come up with a satisfactory one.
Most natural historians can store their specimens: butterflies on pins, rocks in drawers, pressed plants on sheets of paper. The collector of rainbows has only words and pictures to go on. If you shine artificial light through a man-made spray you can measure refraction, reflection and the interference between incident and reflected rays precisely, but experiments do not replicate or explain everything you see in the field, which is where rainbow behaviour, like animal behaviour, must be studied. There, too, rainbows are different from most natural phenomena. Accounts of volcanic eruptions, the migration of birds and the occurrence of fossils are tested by quantitative measurement and statistics; rainbow theory is tested by comparing precisely measured angles with those that mathematical physics predicts. You quickly get into difficult stuff. For example, theories about the nature of light – is it made up of waves or corpuscles? – were challenged by observations of rainbows. A letter from Benjamin Langwith, Rector of Petworth, in the Royal Society’s Philosophical Transactions of 1723, describes supernumerary rainbows – the fainter and still fainter repeats of the primary bow which can sometimes be seen to lie within its arc:
The first series of colours was as usual, only the purple had a far greater mixture of red in it than I had ever seen in the prismatic purple; under this was a coloured arch, in which the green was so predominant, that I could not distinguish either the yellow or the blue: still lower was an arch of purple, like the former, highly saturated with red, under which I could not distinguish any more colours.
Langwith then went on, as Raymond Lee and Alistair Fraser put it, to question the rainbow wisdom of the Royal Society’s President, Isaac Newton: ‘I begin now to imagine, that the Rainbow seldom appears very lively without something of this Nature, and that the suppos’d exact Agreement between the Colours of the Rainbow and those of the Prism, is the reason that it has been so little observed.’ What he accurately observed did indeed require an explanation. It came only much later, in the 19th century, when wave theory suggested that the incident and reflected rays within a water drop set up interference patterns.
It is not surprising that the Greeks, who had mastered so much of geometry, should have had some insight into the basic structure of the rainbow – Aristotle described it as a reflection which took its arcuate shape from the curved surface of an upturned hemispherical bowl of cloud. Almost every subsequent advance in our understanding of the physics of light and the physiology of sight has made it possible to say something new about the rainbow. Even now, there are no photographs of the ‘tertiary’ rainbow, which appears behind you as you look at the primary bow, although there are good accounts of it having been seen in the wild.