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The whole page is taken up with the following notes:
Gerrard Leir[...]ss, in the principles | of Painting has asserted that | lights are always parrell <...> | and gives as an instance by the | Suns declining rays that no undr | part of a projection can receive the | Sun’s ray ... by the extreme dis- | tance and by the Theory that all | Lights are parralel only concludes | that never can appear to diverge | but when the rays of the sun enter | ing an aperture gives the size of that | aperture by an opposite plane, | but that is only point Blank, for | instance the rainbow gives and | illu[?mines] a wall immediately oposed | by immediately the sun rises or | sinks below the centre of the aperture | it increases because the plane
This is the beginning of a passage which continues on the recto of the leaf (D07510) and on further pages back to folio 82 verso (D07493). John Gage has discussed these provisional notes (not developed in the perspective lectures) as an example of Turner’s close observation of natural phenomena,1 in this case the question of sunlight travelling in parallel lines or otherwise, responding to The Art of Painting by Gérard de Lairesse (1640–1711),2 in the English translation by John Frederick Frisch (London 1738 and later editions).3 Maurice Davies has registered Turner’s notes as ‘on light and shadow’, as part of a longer sequence running back to folio 72 verso (D07473).4
The key passages of Lairesse’s text (1738 edition) occur in Book V (‘Of Lights and Shades’). Chapter XV is headed: ‘Of the Sun’s Light upon Objects at Rising and Setting’. The chapter opens as follows on pages 229–30 (the italics are as printed):
It is unaccountable in many Artists, who handle an Art, whose Theory is built on Mathematics; its Practice, on Experience; and the Execution on Nature; that they take so little Notice of the three Points wherein lies their Honour; especially in the lighting of Objects in a Sun-set; for the Sun, how low soever, cannot shine on any Object under the Parallels, namely, not in the least from underneath, were the Object, if I may say so, as high as the Clouds; and yet we see many Paintings, wherein the Objects are, by a Sun-set, more lighted from underneath than above: which is contrary to Nature ...