The gleam of a rainbow, the shimmer of a butterfly’s wings, the flashing, shifting colours of pearl shell in sunlight, the sheen glimpsed on the plate-glass windows of an old commentary box at Strathalbyn Oval, the glint in the deep-blue feathers of a riflebird — what joins these multiplicit visual phenomena together, what mysterious property do they share? Iridescence, of course, the play of colours, the spectrum’s mesmerising dance.
The science and the many cultural meanings of iridescence were the subject of a lovely smallscale exhibition held recently at the South Australian Museum. It brought together an unusual range of objects: Chinese filigree carved pearl shells, images of glinting clouds and atmospheric haloes, display cabinets full of multicoloured beetles, dazzling crystals, reptile skins. It was an intensely varied show on variation; it cast a charm on those who saw it — and that charm and attendant complexity are now re-created and intensified in a new study of the subject, a kind of posthumous catalogue, itself a work of utmost artistry, refulgent and beautiful, written jointly by distinguished mineralogist Michael Snow and Peter Sutton, the unclassifiably multifarious Adelaide-based scholar.
The two chart their field in mazy fashion, seeking both to pin down the precise contours of their subject and to explore its most elusive depths. Their museum show looked back to an earlier occasion, now almost legendary — a 1988 seminar on iridescence attended by a group of scientific and art world virtuosi, many of whom came to the event, as invited, wearing something iridescent. “The seminar took place,” writes Sutton, “in the gracious rooms of the Royal Society of South Australia, around a huge table covered in beetles, moths, snakes, shells and other appropriate natural history specimens from the South Australian Museum, and ended with the opening of a carton of champagne — it was a sparkling occasion in more ways than one.”
On first inspection, the gleam of iridescence that we see on spreading oil slicks, in pearls, on the feathers of peacocks and in burnt glass is straightforward enough: it is a trick of light alone, not of pigment or colour; it is the consequence of light rays bouncing off a surface or entering and passing out of liquid media. We can simplify: it is “the change in hue of the colour of an object as the observer changes position, or the object changes position in relation to the viewer, or both”.
Iridescence can be caused by refraction, diffraction, as in the gleam on CDs, or thin film interference, the kind of shimmer we catch in the light on soap bubbles. The term was coined in 1791 by Erasmus Darwin, a poet ancestor of the evolutionist, who called on the sun’s rays to “shake into viewless air the morning dews, / and wave in light their iridescent hues” — but the scientific principle behind the phenomenon had Iridescence: The Play of Colours By Peter Sutton and Michael Snow Thames and Hudson, 160pp, $40 been set out more than a century earlier by the Anglo-Irish chemist Robert Boyle. In his wake came a legion of researchers, from the early investigator of light waves, Thomas Young, down to the father-and-son team of Nobel-winning crystal structure pioneers, William Henry Bragg and the Adelaide-born William Lawrence Bragg. But Sutton and Snow have a rather wider field in view than mere objective definition — their aim is to investigate “what it is