Maurice Davies has identified the source of this diagram and the one on folio 69 recto opposite (D07469) as Heinrich Lautensack, Des Circkels unt Richtscheyts, auch der Perspectiva, Frankfurt 1564, ‘pp 13, 18’.1 There is another taken from the book, which Turner probably consulted at the British Museum (since transferred to the British Library, London), on folio 45 recto (D07431).
The present sketch is from plate 6 on folio 13 recto. In Lautensack’s diagram, three symmetrical quadrilaterals with sloping sides rise superimposed from the same base at the lower centre, with the line they rest on and the semi-circle above them indicated by solid lines, while the various other radiating and horizontal lines are dotted. Turner’s somewhat inaccurate transcription introduces a spurious diagonal from the lower left, as well as producing lines from the quardrilaterals towards the upper centre to form a triangle. The point demonstrated by the original, unlabelled diagram is unclear.
The bottom (outer) half of the leaf has been torn off irregularly but fairly neatly, reducing the height from the usual 115 mm. Ruskin’s 1850s page number is inscribed on the recto (D07467) at the corner next to the torn edge, suggesting that the missing half had been removed by then, presumably by Turner himself. There is some offsetting from the ink inscription opposite on folio 69 recto (D07469).
Davies 1994, p.289.