An American text-book of physiology . ptical constants of the eye are subject to considerable individual variation,and that the statements of authors concerning them are not always refracting surfaces of the eye may be regarded as still further sim-plified, and a so-called reduced eye constructed which is very useful forpurposes of optical calculation. This reduced eye, which for optical purposesis the equivalent of the actual eye, is regarded as consisting of a single refract-ing medium having an index of , a radius of curvature of milli-meters, its principal point 2.
An American text-book of physiology . ptical constants of the eye are subject to considerable individual variation,and that the statements of authors concerning them are not always refracting surfaces of the eye may be regarded as still further sim-plified, and a so-called reduced eye constructed which is very useful forpurposes of optical calculation. This reduced eye, which for optical purposesis the equivalent of the actual eye, is regarded as consisting of a single refract-ing medium having an index of , a radius of curvature of milli-meters, its principal point millimeters behind the anterior surface of thecornea, and its nodal point millimeter in front of the posterior surfaceof the lens.^ The principal foci of the reduced eye are respectively in front of and millimeters behind the anterior surface ofthe cornea. Its optical power is equal to dioptrics.^ The position of thisimaginary refracting surface is indicated by the dotted line in figure 216. The. Fig. 216.—Diagram of the formation of a retinal image (after Foster). nodal point, n, in this construction may be regarded as the crossing-point of allthe j)rincipal rays which enter the eye, and, as these rays are unchanged in theirdirection by refraction, it is evident that the image of the point whence theyproceed will be formed at the point where they strike the retina. Hence todetermine the size and position of the retinal image of any external object—e. q. the arrow in Figure 216—it is only necessary to draw lines from various 1 Strictly speaking, there are in this imaginary refracting apparatus which is regarded asequivalent to the actual eye two principal and two nodal points, each pair about millimeterapart. The distance is so small that the two points may, for all ordinary constructions, beregarded as coincident. ^ The optical power of a lens is the reciprocal of its focal length. The dioptry or unit ofoptical power is the powe
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