Port Eliot Red Book: Axis of Vision
AXIS OF VISION
Amongst those numerous causes which are here said to operate independent of Mathematical principles, I will venture to assign one, proceeding from the physical formation of the Eye.
The human Eye is so placed as, by its own motion, to view a certain quantity of the hemisphere without any motion of the head, but the exact quantity has been differently stated by different authors from sixty to ninety degrees. The question before us relates to the height, and not the general magnitude of the object, but it is also capable of seeing much more below its axis than above it, as shewn by the following profile. From hence it appears that the projection of the frontal bone and eyebrow causes great difference betwixt the angle A.B. and the angle A.C. and that the line parallel to the horizon A. (which I shall call the Axis of vision) does not fall in the centre of the opening betwixt the extreme rays B. and C.
For ease of reading, punctuation and capitalization have been modernized.