Page:Handbook of Ophthalmology (3rd edition).djvu/104

 98 OPHTHALMOSCOPE, UPRIGHT IMAGE. eye A be 2 inches, the concave lens must have a focal distance of 8 inches. The fundus is seen just as is an object through a Briicke lens. The convex glass of the instrument corresponds with the refracting apparatus of the myopic eye, while the ocular corresponds with the concave glass in front of the eye of the observer. It follows from what has been said that in high degrees of myopia the image may be formed within a very few inches of the

eye. If, for instance, in Fig. 22, the far point r of the eye be at a distance of 3 inches, an inverted enlarged image, n', will be formed at that distance. The observer, whose axis of vision is directed upon n, needs, in order to see the image at 7i', only to withdraw so far from it that he can accommodate his eye upon it, or he may use a weak convex lens, say -f- -^. He can thus get an inverted enlarged view of any part of the fundus. If the observer approaches the observed eye, the inverted image becomes indistinct so soon as it comes to lie between the observer's eye and his near point. As he approaches still nearer it becomes unrecognizable. The extent of the visual field depends upon the size of the pupil of the eye observed. But even with the pupil fully dilated it must always be rather small, since the distance between the examiner's and the examined eye must necessarily be from 8 to 12 inches. If the examined eye is hypermetropic,—that is, if the fundus lie

in front of the focus of the dioptric apparatus,—then will the light reflected from a (Fig. 23) diverge, after its exit from the eye, as if