A text-book of physiology, for medical students and physicians . lens. The curved surfaces of these t ransparent, bodies nbstanfially like ;i convex glass lens, and the physics of the formation of an image by such a lens may be used to explain the refractive processes in the eye. To understand the formation image by a biconvex lens the following physical facts must he 298 DIOPTRICS OF THE EYE. 299 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. 123). This focus for parallel rays is the principal f
A text-book of physiology, for medical students and physicians . lens. The curved surfaces of these t ransparent, bodies nbstanfially like ;i convex glass lens, and the physics of the formation of an image by such a lens may be used to explain the refractive processes in the eye. To understand the formation image by a biconvex lens the following physical facts must he 298 DIOPTRICS OF THE EYE. 299 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. 123). 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. 123.—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. On the other hand, if a luminous object is placed at F therays from it that strike upon the lens will emerge from the othersurface as parallel rays of light. If a luminous point (/, Fig. 123)is placed in front of such a lens at a distance greater than theprincipal focal distance, but not so far as to give practicallyparallel rays, the cone of diverging rays from it that impingesupon the surface of the lens will be brought to a focus (/) furtheraway than the principal focus. Conversely the rays from aluminous point at / will be brought to a focus at /. These points,/ and /, are therefore spoken of as conjugate foci. All luminous 300 THE SPECIAL SENSES. points within the limits
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