Port Eliot Red Book: Axis of Vision (Conclusion)
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 I believe it will be generally found that 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 do not distinguish objects more than 28 degrees above my axis of vision, altho' I can distinctly see them 57 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 30 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. To this Theory it may perhaps be objected that, in the act of seeing, the motion of the head is too rapid to affect any material difference, but it will be found by making the experiment attentively that the object is seen in a new point of view from the instant the head is moved, because the rays no longer meet at the same centre, and therefore the effect of such vision on the mind is rather a renewal in succession of similar ideas than the same single idea simultaneously excited: and this difference may be compared to that betwixt seeing a Landscape reflected in a mirror at rest and the same landscape when the mirror has been moved from its original position.
For ease of reading, punctuation and capitalization have been modernized.