When you wish to see whether the general effect of your picture corresponds with that of the object represented after nature, take a mirror and set it so that it reflects the actual thing, and then compare the reflection with your picture, and consider carefully whether the subject of the two images is in conformity with both, studying especially the mirror. The mirror ought to be taken as a guide... you see the picture made upon one plane showing things which appear in relief, and the mirror upon one plane does the same. The picture is on one single surface, and the mirror is the same.... if you but know well how to compose your picture it will also seem a natural thing seen in a great mirror.

The Notebooks of Leonardo Da Vinci (MacCurdy, 1938) - XXIX Precepts of the Painter