Prepared in connection with his lectures as Professor of Perspective at the Royal Academy, Turner’s diagram is based on an illustration from Samuel Cunn’s Euclid’s Elements of Geometry (London 1759, Plane Trigonometry). It represents Proposition 7 on Plane Trigonometry, which Cunn defines as ‘Theorem. In small arcs, the sines and tangents of the same arcs are nearly to one another, in a ratio of equality’.1 There are sketches of the Proposition in Turner’s lecture notes.2
Blank, save for an inscription by an unknown hand in pencil ‘25’ bottom left.
Supported by The Samuel H. Kress Foundation