On Smoke

Experience shows us that the air must have darkness beyond it and yet it appears blue. If you produce a small quantity of smoke from dry wood and the rays of the sun fall on this smoke and if you then place behind the smoke a piece of black velvet on which the sun does not shine, you will see that all the smoke which is between the eye and the black stuff will appear of a beautiful blue colour. And if instead of the velvet you place a white cloth smoke, that is too thick smoke, hinders and too thin smoke does not produce, the perfection of this blue colour. Hence a moderate amount of smoke produces the finest blue.

~ Leonardo da Vinci (1452-04-15 1519-05-02 age:67)