Optical indicatrix

'It may look like a potato with sticks in it, but this optical indicatrix model was instrumental in working out the mathematics of optics in mineral microscopy.'
Mike Rumsey, mineralogist

This putty ellipsoid was made by Sir Lazarus Fletcher, Keeper of Mineralogy (1880-1909) at the Natural History Museum. It represents the optical indicatrix, a geometric figure used to describe how light is transmitted by a crystal. The two sticks marked in blue can be moved along a groove, depending on the mineral being modelled. The other three sticks are fixed in place.

The model was part of early work on optical phenomena associated with crystals. Studies revealed that the optical properties of crystals were fundamentally linked to their symmetry and physical properties.

One such optical phenomenon is double refraction, where a light ray entering a crystal is split into two rays. The earliest observations of this were in the mineral calcite. When an object is viewed through a well-formed calcite crystal, it appears as two identical images side-by-side. If the crystal is rotated, only one image moves, and at a specific angle the two images merge.

A key development in explaining phenomena such as double refraction was understanding the link between the optical properties of crystals and their symmetry. These studies revealed that the seven mineral crystal systems fell into three optical classes: isotropic, uniaxial and biaxial. Out of these careful observations came the science of crystal optics, inspiring the invention of many scientific and everyday instruments, including polarisers and optical switches.

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