The Da Vinci Notebooks at sacred-texts.com
No second object can be so much lower than the first as that the eye will not see it higher than the first, if the eye is above the second.
And this second object will never be so much higher than the first as that the eye, being below them, will not see the second as lower than the first.
If the eye sees a second square through the centre of a smaller one, that is nearer, the second, larger square will appear to be surrounded by the smaller one.
Objects that are farther off can never be so large but that those in front, though smaller, will conceal or surround them.
This proposition can be proved by experiment. For if you look through a small hole there is nothing so large that it cannot be seen through it and the object so seen appears surrounded and enclosed by the outline of the sides of the hole. And if you stop it up, this small stopping will conceal the view of the largest object.