THE SENSE OF SIGHT 



705 



mm. before the anterior surface of the cornea; the second focal point is 

 situated 15.619 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 1 and H v there- 

 fore amounts to 15.498 mm., and the posterior focal distance, H 2 to F 2 , to 

 20.713 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 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 



FIG. 292. DIAGRAM SHOWING THE POSITION OF THE CARDINAL POINTS IN THE "SCHEMATIC 

 EYE." The continuous lines in the upper half of the figure show their position in the passive 

 emmetropic eye. The dotted lines indicate the change in their position in an eye accommodated 

 for the object A at the distance a from the cornea, or 152 mm. The lower half of the figure shows 

 the formation of a distinct image on the retina of an eye accommodated for the object A at the 

 distance a from the cornea. 



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 

 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 



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