Maurice Davies has not identified the source of this diagram, dated 1560; it is one of several copied into the Perspective sketchbook which Turner did not use for his perspective lectures.1 He reads the last word as ‘?shadow’, and suggests the method shown may be for placing a cube in perspective, or relate to shadows or reflections.2
In fact, the diagram is from Jean Cousin, Liure de Perspectiue, Paris 1560, folio Giv verso, headed ‘Reigle pour figures qui ne par le front sont veues’, i.e. a perspective method for objects not seen face on. The position of Turner’s inscription is misleading, as the drawing should be viewed with the page turned the other way up and with the horizontal line within the diagram towards the bottom. The original diagram focuses on a rectangular block above the horizontal line marked ‘Corps | esleué’ (i.e. elevé or raised), with vanishing points produced above, and a plan projected below the horizontal line. Further vanishing points produced far to the left and right have not been copied here. The central section of the plate is heavily annotated with numbers and letters, and the plan below marked at successive descending levels variously ‘Plan’ / ‘Perspectif’, ‘Ligne’ / ‘Terre’, and ‘Plan’ / ‘Geometrial’.
An explanation follows on folios Giv verso and H[i] recto of Cousin’s French text. Turner presumably consulted the copy at the British Museum (since transferred to the British Library, London; see the sketchbook Introduction).
At some point, Turner acquired his own copy of Joseph Moxon’s Practical Perspective, or Perspective Made Easy ..., (London 1670),3 to which he appears to refer in his note.