Page:A Treatise on Painting.djvu/309

 first the thickness of air, the colour H is placed in four degrees of thickness, the colour G in two degrees, and E at one degree. Now let us see whether the distances are in an equal inverse proportion; the colour E is at two degrees and a half of distance, G at two degrees, and H at one degree. But as this distance has not an exact proportion with the thickness of air, it is necessary to make a third calculation in this manner: A C is perfectly like and equal to A F; the half degree, C B, is like but not equal to A F, because it is only half a degree in length, which is equal to a whole degree of the quality of the air above; so that by this calculation we shall solve the question. For A C is equal to two degrees of thickness of the air above, and the half degree C B is equal to a whole degree of the same air above; and one degree more is to be taken in, viz. B E, which makes the fourth. A H has four degrees of thickness of air, A G also four, viz. A F two in value, and F G also two, which taken together make four. A E has also four, because A C contains two, and C D one, which is the half of A C, and in the same quality of air; and there is a whole degree above in the thin air, which all together make four. So that if A E is not double the distance A G, nor four times the distance A H, it is made equivalent by the half degree C B of thick air, which is equal to a whole degree of thin air above. This proves the truth of the proposition, that the colour H G E does not undergo any alteration by these different distances. Rh