Maurice William Davies, ‘J.M.W. Turner’s Approach to Perspective in his Royal Academy Lectures of 1811’, unpublished Ph.D thesis, Courtauld Institute of Art, London 1994, p.289
Maurice Davies has not identified the source of this diagram,1 presumably taken from a published treatise. It appears to be a method of showing a circle in perspective. In view of these uncertainties, the correct orientation of the drawing is unclear.
Davies 1994, p.289.