Doubtless these angles may vary in different individuals, from various causes, such as the prominency of the eye, the habit or usual position of the head, &c. yet the upper angle A B will seldom be greater than one half of the lower angle A C; and I have ascertained, with some precision, that I could not distinguish objects more than twenty-eight degrees above my axis of vision, although I can distinctly see them fifty-seven degrees below it. From hence I conclude, that the distance at which an object appears at its greatest height, is, when the axis of vision and the summit of the object form an angle of about thirty degrees; because, under this angle, the eye perceives its full extent without moving the head, yet not without some effort of the eye itself to comprehend the whole of the object.
