An ophthalmic surgeon is “convinced JMW Turner was slightly colour-blind, and this particularly affected his perception of red and blue. The blues are all wrong, either too dark or too bright, and the reds get stronger and stronger, which is exactly what you would expect. And I have no doubt that later in life he had untreated cataracts, which would have made the centre of his field of vision very blurred, with some objects at the edges in focus – and with exactly that effect of dazzling shimmering light we see in the paintings.”