THE SENSE OF SIGHT. 



653 



The Schematic Eye. Accepting the system of cardinal points, Listing, 

 Bonders, and v. Helmholtz have constructed " schematic" eyes to be sub- 

 stituted for the refracting system of the natural eye. 



For this purpose it is necessary to make use of the various estimates of 

 the indices of refraction of the different media, of the radii of curvatures 

 of the different refracting surfaces, and of the distances separating them, to 

 deduce an average eye as a basis for calculation. The most widely accepted 

 attempt is that of v. Helmholtz. The data he assumed are as follows : The 

 refractive index of air = i; of the cornea and aqueous humor, 1.3365; of the 

 lens, 1.4371; of the vitreous humor, 1.3365; the radius of curvature of the 

 cornea, 7.829 mm. ; of the anterior surface of the lens, 10 mm. ; of the posterior 

 surface, 6 mm.; the distance from the apex of the cornea to the anterior 

 surface of the lens, 3.6 mm.; thickness of lens, 3.6 mm. From the above- 



FIG. 312. 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. 



mentioned data v. Helmholtz calculated the position of the cardinal points 

 for the eye as follows (see Fig. 312) : The first focal point is situated 13.745 

 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. 



