Vol. I: I. Prolegomena and General Introduction to the Book on Painting: 25.
The Da Vinci Notebooks
Focus of sight.If the eye is required to look at an object placed too near to it, it cannot judge of it well--as happens to a man who tries to see the tip of his nose. Hence, as a general rule, Nature teaches us that an object can never be seen perfectly unless the space between it and the eye is equal, at least, to the length of the face.