A Colour will appear the same, though removed into different places where the Air is of different densities, provided the distance and the density of the Air, be reciprocally proportional; that is, provided the Colour be no more weakened by the distance of the Eye, than its Passage is facilitated by the thinness of the Air: This may be thus proved; suppose the first or the lowest Air to have four Degrees of density, the Colour to be one Degree distant from the Eye, and the second Air, which is higher than the first, to have lost one Degree of its density, and to be only possess'd of three; add one Degree to the distance of the Colour; and when the Air which is still higher, has lost two Degrees of its density, and the Colour has gained two Degrees in distance; then will your first Colour and your third, be perfectly alike: in a word, if your Colour be rais'd so high, as that the Air, there want three Degrees of its density or grossness, and that the Colour be removed to three Degrees of distance, then you may rest assured that the high and distant Colour, will receive a Diminution of Lustre, equal with that of the lower and nearer Colour; because if the Air on high, want three quarters of the density of that below, the Colour at its utmost Altitude, has added three quarters to its Primitive distance from the Eye; which was the thing we intended to prove.