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Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III. Refraction and Accommodation of the Eye. By Chas. H. Helfrich, M. D., Surgeon to the N. Y. Ophthalmic Hospital. Normal Refraction and Accommodation.The dioptric media of a normal or emmetropic eye (cornea, aqueous humor, lens and vitreous humor) have the requisite refractive power to bring parallel rays of light to a focus on the layer of rods and cones of the retina. These media are centered on the optic axis, a line passing through the centre of the cornea and the posterior pole of the eye. Fig. 8. Schematic eye. ', anterior or first principal focus; A, anterior surface of tee cornea; H' and H, principal points; K' and K, nodal points; +, posterior or second principal focus; F.f., fovea centralis; l' 4, optic axis. Upon the optic axis are situated the cardinal points of the dioptric system. Objects situated at a distance of five metres or more are considered as being at infinity, because those rays from them which enter an eye are so slightly divergent that for practical purposes they may be considered parallel. As parallel rays are brought to a focus at the second principal focus, the eye is capable of forming distinct inverted images of distant objects upon the retina. The eye, however, can also see near objects distinctly, and as the rays from such sources become more divergent the nearer they approach, it is obvious that it must contain some mechanism to increase its refractive power. The power by which it is increased so that divergent rays are also brought to a focus on the retina is the accommodation. Fig. 9. Changes in the eye produce: ! by accommodation, r, cornea; z, anterior chamber; /, lens; v, vitreous humor;;', iris;, onula of Zinn; m, ciliary muscle. By the term static refraction is meant the power the eye has when...

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