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Chap. CCLXXXIII.—A Precept of Perspective in regard to Painting.
When, on account of some particular quality of the air, you can no longer distinguish the difference between the lights and shadows of objects, you may reject the perspective of shadows, and make use only of the linear perspective, and the diminution of colours, to lessen the knowledge of the objects opposed to the eye; and this, that is to say, the loss of the knowledge of the figure of each object, will make the same object appear more remote.
The eye can never arrive at a perfect knowledge of the interval between two objects variously distant, by means of the linear perspective alone, if not assisted by the perspective of colours.
Chap. CCLXXXIV.—Of the Perspective of Colours.
The air will participate less of the azure of the sky, in proportion as it comes nearer to the horizon, as it is proved by the third and ninth proposition, that pure and subtile bodies (such as compose the air) will be less illuminated by the sun than those of thicker and grosser substance: and as it is certain that the air which is remote from the earth, is thinner than that which is near it, it will follow, that the latter will be more impregnated with the rays of the sun, which giving light at the same time to an infinity of atoms floating in this air, renders it more sensible to the eye. So that the air will appear lighter towards the horizon, and darker as well as bluer in looking up to the sky; because there is more of the thick air between our eyes and the horizon, than between our eyes and that part of the sky above our heads.