THE SENSE OF SIGHT. 



611 



ray until it meets the first principal plane in C. Draw C D 

 parallel to the axis. Now assume any point, such as E, in the 

 given ray, and find its image E t by the Rule i. Then D E l 

 becomes the course of the refracted ray. 



The Schematic Eye. Accepting the system of cardinal points, 

 Listing, Bonders, and v. Helmholtz have constructed "schematic" 

 eyes to be substituted for the refracting system of the natural eye. 



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

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

 of curvatures of the different refracting surfaces, and of the distances 



PIG. 287. 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. 



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 ante- 

 rior surface of the lens, 3.6 mm.; thickness of lens, 3.6 mm. From 

 the above-mentioned data v. Helmholtz calculated the position of 

 the cardinal points for the eye as follows (see Fig. 287): The first 



