Quotation from: The Notebooks of Leonardo Da Vinci

Written by: Leonardo da Vinci

Therefore when you have sketched the face draw lines passing from
one corner of the eye to the other; and so for the placing of each
feature; and after having drawn the ends of the lines beyond the two
sides of the face, look if the spaces inside the same parallel lines
on the right and on the left are equal [12]. But be sure to remember
to make these lines tend to the point of sight.