...has also 4, because a b holds 2 and one c d, which is the half of a b and of that same air, and one whole is above in the thin air, which makes 4. Therefore, if the distance a e is not double the distance a g, nor quadruple the distance a h, it is restored by the c d, half a degree of thick air, which is worth a whole degree of the thinner air that lies above it. And thus our purpose is concluded, that is, that the colour h g e does not vary through various distances.

[199] a. Of the perspective of colours.

Of one same colour placed at various distances and heights, such will be the proportion of its brightenings as will be that of the distances which each of these colours has from the eye that sees it. It is proved, and let e b c d be one same colour; the first, e, is placed two degrees of distance from the eye a; and the second, which is b, is 4 degrees away; and the 3rd, which is c, is 6 degrees; and the 4th, which is d, is 8 degrees, as the definitions of the circles that cut upon the line show, as is seen above the line a r; then let a r s p be one degree of thin air; s p e t one degree of thicker air: it will follow that the first colour e will pass to the eye through one degree of thick air, e s, and through one degree of less thick air, s a, and the colour b will send its like-...