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Maurice Davies has identified the source of the diagram as Giulio Troili, Paradossi per Pratticare la Prospettiva senza Saperla, Bologna 1683, page 79, fig.2. Most of the inscription below it comes from the book’s full title,1 which continues: ‘fiori, per facilitare l’intelligenza, frutti, per non operare alla cieca’. The whole can be roughly translated from the Italian as ‘Paradoxes for practising perspective without knowing it: the flowers, in order to facilitate intelligence; the fruits, in order not to operate completely without information’.
Turner probably consulted the copy at the British Museum (since transferred to the British Library, London). He appears to have made a false start in copying the diagram, placing a diagonal across the whole face of the cube in the abandoned version on the left. There is another diagram apparently from the same source on folio 44 verso (D07430).
Davies 1994, p.289.