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The diagram is not annotated. Maurice Davies has identified it as coming from Viator (Jean Pélerin), De Artificiali Perspectiva, Toul 1505, plate Bii.1 This rather quaint engraving shows a cross with steps all around, in a landscape of distant rolling hills and trees; below is a plan of the concentric steps, seen in perspective, with a ruled horizontal line underneath, as Turner shows.
Andrea Fredericksen has linked Turner’s sketches of Viator’s diagram of a square in perspective on folios 27 recto and 44 recto to his perspective lecture diagram 27, showing concentric squares in perspective (see entry for Tate D17041; Turner Bequest CXCV 71). That diagram also seems to have been informed by the present drawing, combining the diamond shape here with the lines receding to three vanishing points set down on the other pages.
Davies 1994, p.288.
There is some offsetting from the diagram and inscription in ink opposite on folio 27 recto (D07396).