Take a piece of glass of the size of a half sheet of royal folio paper, and fix it... between your eye and the object you wish to portray. Then move it away until your eye is two-thirds of a braccio away from the piece of glass, and fasten your head by means of an instrument in such a way as to prevent any movement of it whatsoever. Then close or cover up one eye, and with a brush or a piece of red chalk finely ground mark out on the glass what is visible beyond it; afterwards, copy it by tracing on paper from the glass, then prick it out upon paper of a better quality and paint it if you so desire, paying special attention to the aerial perspective.

The Notebooks of Leonardo Da Vinci (MacCurdy, 1938) - XXIX Precepts of the Painter