Unpublished letter, 'The theory of vision [part 3]' from W F [William Ford] Stevenson to Samuel Hunter Christie Stevenson asserts his confidence in his theory of vision, and asks Christie to acknowledge receipt of his letters and present them before the Royal Society. Subject: Vision / Optometry Received 6 April 1845. Written by Stevenson at Hôtel Meurice, Paris. 3 April 1845