Query, At what distance does any object appear at its greatest height? The general optical distinction of the magnitude of objects is into real and apparent; the real being what its name imports, and the apparent, not that which may ultimately result to the mind, but that which is immediately impressed on the eye. This is measured by a plain and certain rule, namely, the angle which is formed at the eye, by lines drawn from the extremities of the object. The apparent height of a man, therefore, at a quarter of a mile distance, is not the conception which we form of his height, but the opening or angle of the two lines above-mentioned, viz. of the two drawn from the extremities of the object to our eye. This apparent height, therefore, of any object, will be measured always upon the simplest principles; and will vary according to, first, the distance of the object; secondly, the inclination it makes with the horizon; and, thirdly, our relative elevation or depression. Any two of the above three things continuing the same, the apparent magnitude will decrease with the third, though not in exact proportion to it.
