. Human physiology : designed for colleges and the higher classes in schools, and for general reading. xisted, he wouldhave complained of it quite as readily as he did of the difficultyin estimating the size and the distance of objects. The aboveexplanation of erect vision, and other explanations of a similarcharacter, are based upon a wrong idea of the office which thenerve performs in the process of vision. It is not the imageformed upon the retina which is transmitted to the brain, butan impression produced by that image. The mind does notlook in upon the eye and see the image, but it recei
. Human physiology : designed for colleges and the higher classes in schools, and for general reading. xisted, he wouldhave complained of it quite as readily as he did of the difficultyin estimating the size and the distance of objects. The aboveexplanation of erect vision, and other explanations of a similarcharacter, are based upon a wrong idea of the office which thenerve performs in the process of vision. It is not the imageformed upon the retina which is transmitted to the brain, butan impression produced by that image. The mind does notlook in upon the eye and see the image, but it receives an im-pression from it through the nerve; and this impression is somanaged that the mind gets the right idea of the relative positionof objects. Of the way in which this is done we know as littleas we know of the nature of the impression itself. 452. It is an interesting and wouderful fact, that as we lookat an object with both eyes, although there are two imagesformed, and therefore two impressions are carried to the brainby the two nerves, yet a single impression is produced in the FIG. THE EYE. 301 Correspondence of action between the two eyes. mind. To produce this single effect at the end of the processof seeing, it is manifest that there must be a very exact corres-pondence in the two eyes as optical instruments. The twoimages must be similar, and must be formed on correspondingparts of the retina in both eyes. Thus, if there be a range ofobjects, as at A, B, C, in Fig. 171, the impression will be asingle one in the mind, because the picture of these objects ison the same part of the retina in both eyes, a, 6, c, and a, b\ if you press with your finger one of the eyes a little outof its place, all these objects will appear double, because theirimages occupy different parts of the retina in the two eyes. 453. It is essential, therefore, that the muscles which movethe eyes, as we direct them towards different objects, shouldharmonize in their action. They mu
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