MEMORANDUM OF THINGS I REQUIRE TO HAVE GRANTED [AS AXIOMS] IN MY EXPLANATION OF PERSPECTIVE.
I ask to have this much granted me—to assert that every ray passing through air of equal density throughout, travels in a straight line from its cause to the object or place it falls upon.
FIRST BOOK ON LIGHT AND SHADE.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.