Vol. I: I. Prolegomena and General Introduction to the Book on Painting: 38.
The Da Vinci Notebooks
The pupil of the eye, in the open air, changes in size with every degree of motion from the sun; and at every degree of its changes one and the same object seen by it will appear of a different size; although most frequently the relative scale of surrounding objects does not allow us to detect these variations in any single object we may look at.