[198] Of the colour that shows no variety in various thicknesses of air.

It is possible that one and the same colour makes no change at various distances, and this will happen when the proportion of the thicknesses of the air and the proportions of the distances that the colours have from the eye are one and the same, but converse.

It is proved: let a be the eye, h a colour of whatever you wish, placed at one degree of distance remote from the eye, in air of 4 degrees of thickness; but because the second degree above, m n l, has air half as thin, carrying the same colour into it, it is necessary that such colour be twice as remote from the eye as it was before; therefore we shall place it two degrees away, a f and f g, distant from the eye, and it will be the colour g; which then, rising into the degree of double thinness at the 2nd layer m a n l, which will be the degree o m p n, must be placed at the height e, and will be distant from the eye by the whole line a e, which is proved to be worth, in thickness of air, as much as the distance a g; and it is proved thus: if a g, a distance interposed of one same air between the eye and the colour, occupies 2 degrees, this distance is sufficient to make the colour g, raised to e, not vary in its power, because the degree a b and the degree a f, being one same thickness of air, are alike and equal, and the degree c d, though it be equal in length to the degree f g, is not alike...