Correspondence between John Nye and James Callahan discussing algebreic geometry and water drops under gravity Discussing Callahan's work on the E6 surface singularity in relation to papers by Nye on the caustics of water drops in a gravity free environment and while under the effect of gravity ['Optical caustics in the near field from liquid drops' published in Proceedings of the Royal Society A in 1978, and 'Optical caustics from liquid drops under gravity' published in Philosophical Transactions A in 1979]. 1977-1978