This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901 Excerpt: ... they came from l, the image of the actual source of light, L. We may, for convenience of description, call the image l the immediate source of light, in contradistinction to Z, the actual source of light. It will be seen that the field of illumination is greatest in the hypermetropic eye, and least in the myopic eye, ...
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1901 Excerpt: ... they came from l, the image of the actual source of light, L. We may, for convenience of description, call the image l the immediate source of light, in contradistinction to Z, the actual source of light. It will be seen that the field of illumination is greatest in the hypermetropic eye, and least in the myopic eye, being reduced to a point when the myopia is such that l is the remote point of the eye. In emmetropia the field is intermediate in size. As the mirror is carried farther away from the eye, the divergence of the rays becomes less, and at a great distance approaches parallelism. In this case the rays are practically J brought to a focus on the retina in emmetropia, whereas there is an area of diffusion in hypermetropia, the rays not having yet come to a focus, and also in myopia, the rays having come to a focus in the vitreous and crossed. The size of the area of diffusion is obviously dependent upon the size of the aperture of entry, i.e. the size of the pupil. (b) Hie Concave Mirror.--The focal length of the concave ophthalmoscope mirror is usually about 20 cm. (8 inches). The nature of the rays entering the eye will vary according to the distance of the mirror. If this is more than 20 cm. (Fig. 48), say 30 cm., from the light, the rays will be convergent, as if coming to a focus 60 cm. in front of the mirror. They will be rendered more convergent by the dioptric mechanism of the eye, will cross in the vitreous and form an area of diffusion upon the retina, which will be least in hypermetropia, greatest in myopia, and intermediate in emmetropia. If the mirror is 20 cm. from the light the reflected rays will be parallel, and will come to a focus upon the retina in emmetropia, behind it in hypermetropia, and in the vitreous in myopia. If the mir...
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