Recent advances in ophthalmic science : The Boylston prize essay for 1865 . the retina N, but the actualretina N, lying as It does behind this point, is not in a positionto receive the Image. The result is, that the myopic eye, evenwhen at perfect rest, cannot see distant objects clearly; the onlyrays, in fact, which can form a distinct image upon the retinaN are those which emanate from a comparatively near object,say at A, and which are sensibly divergent. This point Arepresents, therefore, the extreme distance at which distinct visionis possible, and is called the far point, punctum remotis
Recent advances in ophthalmic science : The Boylston prize essay for 1865 . the retina N, but the actualretina N, lying as It does behind this point, is not in a positionto receive the Image. The result is, that the myopic eye, evenwhen at perfect rest, cannot see distant objects clearly; the onlyrays, in fact, which can form a distinct image upon the retinaN are those which emanate from a comparatively near object,say at A, and which are sensibly divergent. This point Arepresents, therefore, the extreme distance at which distinct visionis possible, and is called the far point, punctum remotissi-mum (r). In accommodation, as shown in the lower half of the figure,the increased curvature of the crystalline greatly augments therefractive power of the eye, so as to bring to a focus upon theretina N rays emanating from the very near point P, punctumproxlmum (p), which is considerably nearer the eye than ^nthe normal condition. 112 RECENT ADVANCES IN OPHTHALMIC SCIENCE. Fig. 21. Diagram illustrating the action of the accommodative faculty in the hyper-metropic Figure 21 represents the two halves of a hypermetropic eye,the upper half in a state of rest, the lower in full accommoda-tion. The hypermetropic is shorter than the emmetropic eye, asis indicated by the flattened outline N, the dotted curve N rep-resenting the normal position of the retina. In the upper halfof the figure, showing the eye at rest, the refractive power ofthe cornea and crystalline is just sufficient to bring parallel raysA to a focus at the normal distance of the retina N, but Is notsufficient to form an image at the actual distance W. The onlyrays, therefore, which can converge upon the retina of the hyper-metropic eye in a state of rest are convergent rays A A, suchas no natural object gives off. The exercise of the full accommodative power may be justsufficient to admit of the convergence upon the retina N of par-allel rays A A (in the lower half of the diagram). In whichcase distant vision
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