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The diagram at the bottom left shows two elevations and a plan of a capital in the classical Tuscan order. Above, Turner notes:
Cavaliere Lorenzo Sirigatti
and endless variety of stairs produced by the Diganall and | points of sight but parralell the remarkable point in this | treatise is the circle
To the right he observes ‘the endless and overbearing | confusion of points here | must be of Disadvan[?gtive]’, while between the elements of the diagram he records ‘Chaptr. XXXIII’ as the relevant section of his source, which Maurice Davies identifies as Lorenzo Sirigatti, La Pratica della prospettiva, Venice 1625, plate 32,1 which is inscribed both ‘CAP.XXXIII’ and ‘32’. The author’s epithet ‘Cavaliere’ is taken from the title page. Turner probably consulted the copy at the British Museum (since transferred to the British Library, London). He comments on the ‘endless variety of stairs’ in the treatise, and plates 13–19 indeed show many varieties; there are also diagrams of circles, regular solids and architecture in perspective.
As Davies2 and Andrea Fredericksen have noted, Turner used the sketch for the left-hand part of his perspective lecture diagram 38 (see entry for Tate D17055; Turner Bequest CXCV 85), without apparently understanding the method, finding it over-elaborate and criticising it further in his lectures. The original consists of a capital at the top centre seen from slightly below, with an elevation at the centre left and a half-plan directly below it towards the bottom left.
The ‘confusion of points’ Turner mentions stems from the innumerable dotted lines produced from the elevation and plan towards a notional vanishing point beyond Sirigatti’s diagram to the right. Turner has adapted his source in numerous ways, half-converting the elevation into a perspective view from below, showing part of the perspective rendering to the right rather than above, and adapting the dotted lines, using them instead to show the relationship between the elevation and the plan.
Davies 1992, pp.47, 108 note 67; Davies 1994, p.288 (for Sirigatti’s plate see fig.4.12; and for another sketch by Turner, from his ‘Royal Academy Lectures’, circa 1807–38, Department of Western Manuscripts, British Library, London, ADD MS 46151 F folio 9 recto, see fig.4.14).
Davies 1992, p.47, with Turner’s lecture diagram, fig.43; see also Davies 1994, pp.121–6 and fig.4.14.