"There is another kind of perspective which I call aerial, because by the difference in the atmosphere one is able to distinguish the various distances... and if in painting you wish to make one seem further away than another you must make the atmosphere somewhat heavy. ...[I]n an atmosphere of uniform density the most distant things seen through it, such as the mountains, in consequence of the great quantity of atmosphere which is between your eye and them, will appear blue... Therefore you should make the building which is nearest... of its natural colour, and that which is more distant make less defined and bluer; and one which you wish should seem as far away again make of double the depth of blue, and one you desire should seem five times as far away make five times as blue."