Note 8 — Chapter II · On Peculiarities in the Perceptions of the Different Senses.

The explanation of our seeing objects erect when the image is inverted has been put very simply, by saying, ‘We call that the lower end of an object which is next the ground.’ The observer cannot look into his own eye; he knows by experience what kind of image corresponds to a man in an upright position. The anatomist tells him that this image is inverted: but this does not disturb the process of judging by experience. It does not appear why any one should be perplexed at the notion of seeing objects erect by means of inverted images, rather than at the notion of seeing objects large by means of small images; or cubical and pyramidal, by means of images on a spherical surface; or green and red, by means of images on a black surface. Indeed some persons have contrived to perplex themselves with these latter questions, as well as the first. The above explanation is not at all affected, as to its substance, if we adopt Sir David Brewster’s expression, and say that the line of visible direction is a line passing through the center of the spherical surface of the retina, and therefore of course perpendicular to the surface. In speaking of ‘the inverted image,’ it has always been supposed to be determined by such lines; and though the point where they intersect may not have been ascertained with exactness by previous physiologists, the philosophical view of the matter was not in any degree vitiated by this imperfection.