612 TEXT-BOOK OF PHYSIOLOGY. 



focal point is situated 13.745 mm. before the anterior surface of the 

 cornea; the posterior focal point is situated 15.6 19 mm. behind the 

 posterior surface of the lens; the first principal point, 1.753 mm - 

 behind the cornea; the second principal point, 2.106 mm. behind the 

 cornea; the first and second nodal points, 6.968 and 7.321 mm. 

 behind the apex of the cornea, respectively. The anterior focal 

 distance of this schematic eye, the distance between F^ and H v 

 therefore amounts to 15.498 mm., and the posterior focal distance, 

 H 2 to F 2 , to 20.7 13 mm. 



When the eye, however, is accommodated for near vision, the 

 relations of the cardinal points are changed and will be as follows, 

 if the point accommodated for, lies 152 mm. from the cornea: 

 Anterior focal distance, 13.990 mm.; posterior focal distance, 18.689 

 mm. ; distance from cornea of the first and second principal points, 

 1.858 and 2.257 mm. respectively; distance of the posterior focus, 

 20.955 mm. from cornea. Given this schematic eye in the accom- 

 modated state, the course of the rays and the determination of the 



position of an image in the last medium 

 of a luminous point in the first can 

 easily be determined by the rules already 

 given. 



The Reduced Eye. As suggested by 

 Listing, this schematic eye may be yet 

 further simplified or reduced to a single 

 FIG. 288. THE REDUCED EYE. refracting surface bounded anteriorly by 



air and posteriorly only by aqueous or 



vitreous humor. Without introducing any noticeable error in the 

 determination of the size of the retinal image, the anterior principal 

 and the anterior nodal points may be disregarded, owing to the minute- 

 ness of the distances (0.39 mm.) separating the two systems of points. 

 There is thus obtained one principal point and one nodal point, 

 which latter becomes the center of curvature of the single refracting 

 surface. The dimensions of this "reduced" eye are as follows (see 

 Fig. 288) : From the anterior surface of the cornea, corresponding to 

 the principal plane, to the nodal point, 5.215 mm.; the anterior focal 

 distance, 15.498 mm.; the posterior focal distance, 20.713 mm.; the 

 index of refraction is 1.3365. There is thus substituted for the 

 natural eye a single refracting surface with a radius of curvature of 

 5 ; 2i5 mm. In such an eye luminous rays emanating from the ante- 

 rior focal point are parallel to the axis after refraction in the interior 

 of the eye. Also rays parallel to the axis before refraction unite at 

 the posterior focal point. 



By means of this reduced eye the construction of the refracted ray, 

 the various calculations as to the size of the image, the size of diffusion 

 circles, etc., are greatly facilitated: e. g., 



