Spherical lenses suffer from optical aberration, which prevents all rays from being focused to a single point. As long ago as 1854, James Clerk Maxwell proposed a mathematical solution to this problem by making the refractive index decrease parabolically from the centre to the edge. Although hard to implement in practice, Alison Sweeney and colleagues at the University of Pennsylvania in the US have now shown that squid already perform this trick via assemblies of transparent S-crystallin protein dimers that vary in size to achieve what Maxwell suggested. The stability of the gradient index of refraction is remarkable, suggesting an anisotropic interaction between the proteins to maintain it, and the finding suggests new ways to make aberration-free lenses.