The apparent height of a body, as upon the same principles any other of its dimensions, is a matter of easy consideration; its inclination, its distance, and the relative position of the observer being known. The difficulty is to know what the conception is that we shall form of the height and magnitude of an object; according to different circumstances, its apparent height, as well as its real height, remaining the same. This, you will see, belongs to wholly different principles, and such as cannot be reduced to certain rules; it appears, too, from hence, that the question has little or nothing to do with mathematical principles, at least beyond those simple ones which I have just stated. Of other principles, the consideration is more diversified: much may be ascribed to the habit, which we probably have, of estimating the height of objects, not by the angle, formed by lines to the summit and the base, when the base is below us, but by that formed between a line from the summit and a line parallel to the horizon; in this way our conception of the magnitude may be less, while the apparent magnitude may be greater. A thousand other causes may likewise operate, amongst which will be some that belong to what is called aerial perspective, or those rules by which we judge of the distance or dimensions of objects, not by their outline on the retina, but by their colour and distinctness. The existence and operation of these can hardly be found, but by a careful examination and comparison of particular instances.
