The Da Vinci Notebooks
Although the objects seen by the eye do, in fact, touch each other as they recede, I will nevertheless found my rule on spaces of 20
braccia each; as a musician does with notes, which, though they can be carried on one into the next, he divides into degrees from note to note calling them 1st, 2nd, 3rd, 4th, 5th; and has affixed a name to each degree in raising or lowering the voice.