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, Spherical Trigonometry). It illustrates Proposition 22 on Spherical Trigonometry: ‘In any spherical triangle A B C, if the angles at the base B and C be of the same affection, then the perpendicular falls within the triangle; and if they be of a different affection, the perpendicular falls without the triangle’.1 There is a sketch of the left diagram in Turner’s lecture notes.2
Blank, save for an inscription by an unknown hand in pencil ‘21’ bottom left.
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