. A text-book of physiology : for medical students and physicians . e lens. The curved surfaces of these transparent bodiesact substantially like a convex glass lens, and the physics of theformation of an image by such a lens may be used to explain therefractive processes in the eye. To understand the formationof an image by a biconvex lens the following physical facts must be 306 DIOPTRICS OF THE EYE. 307 borne in mind. Parallel rays of light falling upon one surface of thelens are brought to a point or focus (F) behind the other surface(Fig. 124). This focus for parallel rays is the principa
. A text-book of physiology : for medical students and physicians . e lens. The curved surfaces of these transparent bodiesact substantially like a convex glass lens, and the physics of theformation of an image by such a lens may be used to explain therefractive processes in the eye. To understand the formationof an image by a biconvex lens the following physical facts must be 306 DIOPTRICS OF THE EYE. 307 borne in mind. Parallel rays of light falling upon one surface of thelens are brought to a point or focus (F) behind the other surface(Fig. 124). This focus for parallel rays is the principal focus andthe distance of this point from the lens is the principal focal dis-tance. This distance depends upon the curvature of the lens andits refractive power, as measured by the refractive index of thematerial of which it is composed. Parallel rays are given theo-retically by a source of light at an infinite distance in front of thelens, but practically objects not nearer than about twenty feetgive rays so little divergent that they may be considered as par-. Fig. 124.—Diagrams to illustrate the refraction of light by a convex lens : a., Refrac-tion of parallel rays ; b., refraction of divergent rays ; c, refraction of divergent rays froma luminous point nearer than the principal focal distance. allel, so far as the optical apparatus of the eye is concerned. Onthe other hand, if a luminous object is placed at F the rays fromit that strike upon the lens will emerge from the other surfaceas parallel rays of light. If a luminous point (/, Fig. 124) isplaced in front of such a lens at a distance greater than the prin-cipal focal distance, but not so far as to give practically parallelrays, the cone of diverging rays from it that impinges upon thesurface of the lens will be brought to a focus (/) further awaythan the principal focus. Conversely, the rays from a lumin-ous point at / will be brought to a focus at /. These points,/ and/, are, therefore, spoken of as conjugate fo
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