Ni­co­las Roth­well

The Weekend Australian - Review - - Books -

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

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