separately presentable. It is a sufficient explanation to say, that we do not see the image on the retina, but see by _means_ of it. The perplexity does not require much more skill to disentangle, than it does to see that a word written in _black_ ink, may signify _white_[8\4]. [Note 8\4: 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.] 8. _Second Paradox of Vision. Single Vision._--(1.) _Small or Distant Objects._--The other difficulty, why with two images on the retina we see only one object, is of a much more real and important kind. This effect is manifestly limited by certain circumstances of a very precise nature; for if we direct our eyes at an object which is very near the eye, we see {310} all other objects double. The fact is not, therefore, that we are incapable of receiving two impressions from the two images, but that, under certain conditions, the two impressions form one. A little attention shows us that these conditions are, that with both eyes we should look at the same object; and again, we find that to look at an object with either eye, is to direct the eye so that the image falls on or near a particular point about the middle of the retina. Thus these middle points in the two retinas correspond, and we see an image single when the two images fall on the corresponding points. Again, as each eye judges of position, and as the two eyes judge similarly, an object will be seen in the same place by one eye and by the other, when the two images which it produces are _similarly situated_ with regard to the _corresponding points_ of the retina[9\4].