Page:A Treatise on Painting.djvu/326

  nearer the air is to water, or to the ground, the thicker it becomes. It is proved by the nineteenth proposition of the second book, that bodies rise in proportion to their weight; and it follows, that a light body will rise higher than another which is heavy.   different objects equal in magnitude, form, shade, and distance from the eye, those will appear the smaller that are placed on the lighter ground. This is exemplified by observing the sun when seen behind a tree without leaves; all the ramifications seen against that great light are so diminished that they remain almost invisible. The same may be observed of a pole placed between the sun and the eye.

Parallel bodies placed upright, and seen through a fog, will appear larger at top than at bottom. This is proved by the ninth proposition, which says, that a fog, or thick air, penetrated by the rays of the sun, will appear whiter the lower they are.  Rh