A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
The Painters are apt to lament themselves, and quarrel with their own Performances, because in copying from the Life, they cannot give their Figures the same Force and Relievo, with which Images appear in a Mirrour; urging that they have Colours of greater Lustre, and Shadows much deeper than any the Mirrour exhibits; and laying the whole blame of their Failure, upon their own Ignorance, or Unhap piness in the Management of them; but they herein abuse themselves, and impute that to their own Weakness, which is an Effect purely Natural: A painted Figure must of necessity appear with less Relievo, than a Figure seen in a Mirrour, (tho' both superficial) unless both the one and the other be only viewed with a single Eye; the Reason is this: The two Eyes, A B, [Tab. 1. Fig. 2.] viewing the two Objects, N M one behind another, M cannot entirely intercept the Sight of N, the Base of the visual Rays being so large, that the farther Object discovers it self beyond the first; but if you only make use of one Eye, as S, [Tab. 1. Fig. 3.] the Object F will intercept the whole Extent of R, because the Pyramid of Visual Rays, issuing from a Point, has the first Body F for its Base; by which means the secondR, of the same size, is entirely hidden*.
* Leonardo is a little obscure in this Chapter, and may, perhaps have been mistaken; the Matter, in a few Words, seems to be this: Every Painting, is a piece of Perspective, and the Figures in it, capable of appearing with as much Relievo, as the natural Objects they represent. But the Figures in Painting are all flat, so that we cannot turn round them, to view their different Sides; there being properly but one Point of View, from whence they may be well seen; whereas we survey all the sides of Natural Bodies; and they always appear with the Relievo they really have.