When on any occasion, you find your self unable to discover any difference, in the brightness of the Lights, or in the obscurity of the Shadows of an Object, that you wou'd imitate, in that case, you may set aside the Perspective of Colours, and only make use of the Lineal Perspective, to diminish the Figures in proportion to their distances; and of the Aerial Perspective, to diminish and weaken their evidency, by showing them less finished, and distinct.

The Eye will never discover the interval between two Objects, differently distant, by means of the mere Lineal Perspective; unless further aided by the Reasoning deduced from the Aerial Perspective.