Carswell was Professor of Morbid Anatomy at University College, London, and one of the leading English pathologists of his day. A fine artist, he personally painted 2,000 water-colours of pathological specimens. His great pathological atlas contains splendid hand-coloured lithographs which he selected from his collection of water-colours and personally drew on stone. Among the many pathologies illustrated was one of the first images of multiple sclerosis.