A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
The lower Bounds and Extremities of far distant Objects, will be less visible than the upper: This is very observable in Mountains, whose Tops have for their Ground, the Sides of some other Mountains, rising behind them; For here, the Basis being incompassed with a Grosser, and more illumined Air, must of course be less distinct and determined than the Summet; so that the Top will be very evident and discernible, the Root all the while being dimm and indistinguishible. The same things happens with regard to Trees, Buildings and all other Bodies rising high into the Air; and hence it is, that looking from a great Distance, at any very tall Tower, we see it larger at the Top, than the Bottom; the thin and less lucid Air, wherewith the Top is surrounded, leaving more Room for the Minute Parts to appear, than the Grosser, and more luminous Medium, investing the Foot of the same Building; as I have elsewhere shewn, on this Principle; that a Gross Air diffusing a Whiteness on Objects, enfeeble their Images; whereas a more subtile Air, in tinging Objects with Azure, takes off less of their Force; and weakens their Impression, but in a less Degree. Of this we have a very sensible Instance in Fortifications; wherein, the Intervals between the Battlements, and the extent of the Battlements themselves, are mutually equal; and yet at a moderate Distance from the Eye, the Intervals appear considerably larger than the Battlements; at a yet greater Distance the Battlements are extremely diminished; Lastly, The Distance is sometimes so Great, that the Battlements entirely disappear and become invisible; so that the Wall appears full, and even, without any Gaps or Interruptions at all*.
* This Instance is fetch'd from the Antient Fortisications, wherein the Walls being of Stone, and being, likewise bleech'd with the Weather, were usually whiter than Air.