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, Book 1, Plate 1 and Book 4). The left figure illustrates Proposition 5 from Book 1: ‘Theorem: The angles at the base of an isosceles triangle are equal between themselves: And, if the equal sides be produced, the angles under the base shall be equal between themselves’.1 The right figure – inexplicably inscribed by Turner ‘Circle within a Triangle’ – is a portion of Proposition 5 from Book 4: ‘Problem: To describe a circle about a given triangle’.2 See Tate D17003; Turner Bequest CXCV 33 for the remaining portion. There are sketches of each Proposition in Turner’s lecture notes.3
Blank, save for an inscription by an unknown hand in pencil ‘14’ bottom left.
Supported by The Samuel H. Kress Foundation