OF THE DIMINUTION OF OBJECTS AT VARIOUS DISTANCES.
A second object as far distant from the first as the first is from the eye will appear half the size of the first, though they be of the same size really.
OF THE DEGREES OF DIMINUTION.
If you place the vertical plane at one braccio from the eye, the first object, being at a distance of 4 braccia from your eye will diminish to 3/4 of its height at that plane; and if it is 8 braccia from the eye, to 7/8; and if it is 16 braccia off, it will diminish to 15/16 of its height and so on by degrees, as the space doubles the diminution will double.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.