John Dalton, the celebrated chemist, realised that his colour perception differed from that of others, and he left a detailed description of how colours appeared to him. Dalton believed that his vitreous humour possessed an abnormal blue tint, causing his anomalous colour perception, and he gave instructions for his eyes to be examined on his death, to test this hypothesis. His wishes were duly carried out, but no blue coloration was found, and Dalton’s hypothesis was refuted. However, the shrivelled remains of one eye …