Page:A Treatise on Painting.djvu/301

 is a much greater quantity of thick air along the horizontal line P D, than along the oblique P S, or the perpendicular P R.   natural colour of any visible object will be diminished in proportion to the density of any other substance which interposes between that object and the eye.   the colours vanish in proportion as the objects diminish in size, according to the distance.   local colour of such objects as are darker than the air, will appear less dark as they are more remote; and, on the contrary, objects lighter than the air will lose their brightness in proportion to their distance from the eye. In general, all objects that are darker or lighter than the air, are discoloured by distance, which changes their quality, so that the lighter appears darker, and the darker lighter.  Rh