Born on 3 September 1868 he was educated at Bristol University, where he won the Gilchrist Scholarship, at University College, London and at St Bartholomew's Hospital. He became a BSc with honours in physiology at the age of 22, qualifying with the Conjoint diploma the following year. He had from the start displayed a great interest in the physiological aspects of vision and in 1901 was granted a BMA scholarship. In 1903 he delivered an Arris and Gale lecture, in 1904 gained the Middlemore prize in ophthalmology and in 1907 the Nettleship Gold Medal. As a result he was appointed to the staff of University College Hospital and of the Royal London Ophthalmic Hospital, which gave him ample scope to pursue his researches in spite of a busy consulting practice. He was much in demand for service on various ministerial committees dealing with factory lighting, sight tests and the prevention of blindness; he also served on the Glass Workers' Cataract Committee of the Royal Society. During the war of 1914-18 he was consultant ophthalmological surgeon to the Armed Forces with the rank of Colonel, Army Medical Service. In 1919 he served on the Advisory Medical Committee of the Air Ministry and in 1922 that of the Admiralty. In 1921 he received the rare distinction, for a medical man, of being elected a Fellow of the Royal Society. He was a member of the Medical Research Council from 1928 to 1932. In 1936 he was elected President of the Royal Society of Medicine and he was also elected a Fellow of University College. He wrote extensively. His popular textbook *Diseases of the Eye*, first published in 1907, ran to its tenth edition in 1942. A large work in four volumes on the *Pathology of the Eye* published in 1904 to 1908 was still a standard work of reference sixty years later. His books in 1915 on *Colour Vision* and *The Theory of Perception* set the seal on his reputation as a scientist and a philosopher, being described by Lord Adrian as a masterly analysis of facts and unbiased examination of theories with a wide knowledge of the borderlands of neurology and psychology. On 3 September 1948 the *British Journal of Ophthalmology* published a special number on the occasion of his eightieth birthday with appreciations by Lord Adrian, J van der Hoeve and R R James, and the *BMJ* had a leader describing him as the doyen of British ophthalmology. A reception and dinner were given by the Faculty of Ophthalmology and the Ophthalmological Society of the United Kingdom at the Royal College of Surgeons, and his portrait by John Gorlay was presented to him. He was a philosopher and scientist, who was able to apply his ophthalmological erudition in a practical manner for the advancement of his own branch of medicine and the benefit of humanity. He held his opinions tenaciously, but a quiet man he was by nature undemonstrative, so that at first sight one might not fully recognise his great ability. He travelled widely, and his great interest was music. Parsons died on 7 October 1957 in University College Hospital in his ninetieth year. A memorial service was held in St Mary's, Bryanston Square on 18 October.