...its likeness to the eye a through two degrees of thick air, and through two of the less thick; and c will send it through 3 of the thick and 3 of the less thick; and the colour d through 4 of the thick and four of the less thick. And thus we have proved here that such is the proportion of the diminutions of colours, or I would say losses, as is that of their distances from the eye that sees them; and this happens only in colours that are of equal height, because in those that are of unequal heights the same rule is not observed, they being in airs of various thicknesses, which make various occupations upon those colours.

[200] Of the colour that does not change in various thicknesses of air.

A colour placed in different thicknesses of air will not change, when the one is as much more remote from the eye than the other as the air is thinner.

It is proved thus: if the first low air has 4 degrees of thickness, and the colour is distant one degree from the eye, a b, and the 2nd higher air has 3 degrees of thickness, having lost one degree, make the colour gain one degree of distance, a c; and when the higher air has lost 2 degrees of thickness, and the colour gained two degrees of distance, then the first colour is such as is the third, b e d: and to be brief, if the colour rises so much that it enters the air that has lost 3 degrees of thickness, and the colour is moved off 3 degrees of distance, then you can be certain that such loss of colour...