. Human physiology : designed for colleges and the higher classes in schools, and for general reading. e middle pointsof the retina in the two eyes, aand a. The images thus cor-responding in their place on theretina, the impressions earnedfrom them by the two opticnerves to the brain correspoTidalso, and so the vision is the image of the object Bis formed in the two eyes, inparts of the retina that do nutcorrespond, b and b. They areboth on the inside of the mid-dle points, a a, that is towardsthe nose; whereas the outwardpart of the retina in one eyecorresponds with the inward part
. Human physiology : designed for colleges and the higher classes in schools, and for general reading. e middle pointsof the retina in the two eyes, aand a. The images thus cor-responding in their place on theretina, the impressions earnedfrom them by the two opticnerves to the brain correspoTidalso, and so the vision is the image of the object Bis formed in the two eyes, inparts of the retina that do nutcorrespond, b and b. They areboth on the inside of the mid-dle points, a a, that is towardsthe nose; whereas the outwardpart of the retina in one eyecorresponds with the inward part in the other eye, and vice versa. This you will see to betrue by recurring to Fig. 171, in which is shown the way inwhich a row of objects is pictured on the retina in the twoeyes. There you see that the image of the object A, for ex-ample, is in the left eye, L, on the inner side of the middle point,b of the retina; while in the right eye, li, it is at the outer sideof the middle point, b. In the case of the object B, then, in , it is clear to you that the images of it in the two eyes are. THE EYE. 311 Borne of the imagea on the retina are not attended by the mind. formed in parts of the retina that do not correspond, and there-fore it appears double. 465. The application of all this to the point in hand youcan readily see. As the images of all objects in the field ofvision of the two eyes are pictured on the retina, it is plain,according to the facts developed above, that whenever the eyesare directed together to any one object, other objects in thesame direction, but at a different distance, must make imageson the retina in the two eyes that do not correspond. We aretherefore continually seeing double, so far as that part of theprocess of seeing, which consists in the formation of the im-ages, is concerned. But we are not ordinarily conscious ofseeing double. How is this ? How is the difficulty (for it is areal difficulty in the eyes, as a pair of optical instruments
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