512 VISION 



of the prevailing measurements. It must be observed, however, that, as will 

 appear from the table on page 510, the individual variations are considerable. 



In the following table are contained the optical constants from two sche- 

 matic eyes, which have been calculated by Helmholtz on the basis of newer 

 measurements. 



The location is in all cases given as the distance in millimeters from the 

 vertex of the cornea, and is reckoned as positive when it is posterior and 

 negative when anterior. 



Directly Determined 



Refractive index of the aqueous humor and vitreous body. 1 .3376 1 .3365 



Total refractive index of crystalline lens 1 . 4545 1 . 4371 



Radius of curvature of the cornea 8.0 7.829 



Radius of curvature of the anterior surface of the lens 10.0 10.0 



Radius of curvature of the posterior surface of the lens ... 6.0 6.0 



Location of the anterior surface of the lens 3.6 3.6 



Location of the posterior surface of the lens 7.2 7.2 



Calculated 



I. II. 



Cornea : anterior focal distance 23.692 23.266 



Cornea: posterior " " 31.692 31.095 



Lens: focal length 43.707 50.617 



Posterior focal distance of the eye 19.875 20.713 



Anterior " " " " " 14.858 15.498 



Location of the I principal point 1 .9403 1 . 753 



" " II " " 2.3563 2.106 



" " Inodalpoint 6.957 6.968 



" " " II " " 7.373 7.321 



". " " anterior focal point 12.918 13.745 



" " " posterior focal point 22.231 22.819 



In Fig. 210 these values are brought together in a diagram of the human 

 eye enlarged about three times. We see that the principal points (li f h ff ) lie 

 in the middle of the aqueous chamber and the nodal points (k f k f/ ) in the 

 posterior part of the crystalline lens. The posterior focal point F,, falls upon 

 the retina. 



By means of these so-called cardinal points, the path of any given inci- 

 dent ray can be determined, as has been seen on page 511, beyond its last 

 refraction; likewise, the location of any point occurring in the neighborhood 

 of the axis. Since, moreover, the two principal points and the two nodal 

 points lie very close together (by the above table, 0.416 mm. and 0.353 

 mm. apart respectively), for many purposes the two can be regarded as 

 one point and the eye reduced to a single optical system. In this reduced 

 eye the principal point lies (according to Listing's scheme) 2.345 mm. pos- 

 terior to the anterior surface of the cornea and the single nodal point 0.476 

 mm. anterior to the posterior surface of the lens. If from this point a curve 

 be drawn through the reduced principal point (radius of 5.125 mm.) it will 

 represent the anterior limiting surface of the reduced eye; in front of it is 

 air, back of it aqueous humor or the vitreous body. 



As appears also from the above table, the anterior focal distance of the 

 cornea (II eye) is 23.3 mm., that of the entire eye 15.5 and the focal length 



