Lecture Diagram 20: Conic Sections (after Thomas Malton Senior)

In a section on curvilinear perspective in Lecture 2, given as Professor of Perspective at the Royal Academy, Turner addressed the role of conic sections,1 presenting Diagram 20 to illustrate how the theory of these enters into the perspective representation of circles.2 It incorporates elements from several illustrations found in A Compleat Treatise on Perspective in Theory and Practice on the True Principles of Dr Brook Taylor (1775) by the elder Thomas Malton (1726–1801); see pl.VII, figs.29 and 30. Together with the right-hand portion of Diagram 19 (Tate D17032; Turner Bequest CXCV 62), the drawing shows ‘some of the arrangements that can arise when the circle is not on the axis of the eye. In these cases the cone is skewed, or “scalene”’.3 Turner also sketched the diagram in his lecture notes.4

Technical notes:

Peter Bower states that the sheet is Double Elephant size Whatman paper made by William Balston, at Springfield Mill, Maidstone, Kent. The largest group within the perspective drawings, this batch of paper shows a ‘grid-like series of shadows that can be seen within the sheet in transmitted light. This appears to have been caused by a trial method of supporting the woven wire mould cover on the mould’. Because this is the only batch he has seen with such a feature, Bower believes that ‘it may have been tried on one pair of moulds and for some reason never tried again’. He also writes that it is ‘not the best Whatman paper by any means; the weight of this group is also very variable and the moulds have not been kept clean during use’1

Verso:

Blank, save for an inscription by an unknown hand in pencil ‘65’ bottom left.