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TEXT-BOOK OF PHYSIOLOGY. 



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 accom- 

 modated 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 - fr m cornea. Given this schematic eye in the 

 accommodated 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 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 disre- 

 garded, owing to the minuteness 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 re- 

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

 Fig. 313). From the anterior surface of the cornea, corresponding to the 

 principal plane H, to the nodal point N, 5.215 mm., from the anterior focal 

 point F v to the principal plane H, i.e., the anterior focal distance/, 15.498 



FIG. 313. THE REDUCED EYE. 



B 



FIG. 314. THE FORMATION OF AN IMAGE IN THE 

 REDUCED EYE. 



mm.; from the principal plane H to the posterior focal point F 2 , i.e., 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, r, of 5.125 mm. In such an eye luminous rays emanat- 

 ing from the anterior 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., 



In Fig. 314 let A B represent an object. From A a pencil of rays falls 

 on the single refracting surface. One of the rays, the nodal ray, falling on 

 the surface perpendicularly, passes unrefracted through the single nodal point, 

 N, to the posterior focal plane. The remaining rays, partially represented 

 in the figure, falling on this surface under varying degrees of incidence, 

 undergo corresponding degrees of refraction, by which they form a converg- 

 ing cone of rays which unite at a point situated on the nodal ray. These 



