510 



VISION 



lens immersed in aqueous humor. Such a lens does not change the course 

 of the light rays to any appreciable extent. According to an estimate of 

 Helmholtz the focal distance of the cornea embedded in the aqueous humor 

 would be 8.7 m., a distance which, in comparison with the dimensions of the 

 eye, can be regarded as practically infinite. 



The first refracting system is reduced therefore to a simple optical system 

 composed of two media, the air and the aqueous humor, separated by a surface 

 with the curvature of the cornea. 



To be able to follow the course of the light rays in the eye we need there- 

 fore, in addition to the refractive indices already mentioned, only the following 

 data: (1) The radius of curvature of the cornea; (2) the distance of the 

 anterior surface of the lens from the vertex of the cornea; (3) the radius 



FIG. 207. 



of curvature of the anterior surface of the lens; (4) the thickness of the lens 

 and (5) the radius of curvature of the posterior surface of the lens. 



Some of the values found by different authors for these dimensions are 

 given in the following table: 



(1) Radius of curvature of the anterior surface of cornea 6.852-8. 154 mm. 



(2) Distance from vertex of cornea to anterior surface of the lens 2.900-4.09 " 



(3) Distance from vertex of cornea to posterior surface of lens 6.844-7.69 " 



(4) Radius of curvature of anterior surface of lens 7.860-12.58 " 



(5) Radius of curvature of posterior surface of lens 5 . 13 -8.49 " 



To enable us the better to follow light rays through an optical system like 

 that represented by the human eye, let us suppose the refracting surfaces S lt 

 S 2 and $ 3 in Fig. 207 to be related to each other as are the refracting surfaces 

 of the cornea and lens. The points F and F* will be the anterior and posterior 

 focal points of the entire system, the line AA^ its axis. Imagine any incident ray 

 parallel to the axis to be represented by Px. Since all rays parallel to the axis 

 pass through the focal point, whatever course this ray may take through the 

 system, we know that after it is refracted it will pass through the focal point 

 F*. But the incident and refracted rays must meet somewhere if prolonged. 

 Let this point of meeting be e*. Imagine the incident ray Px projected toward 

 Q and call the portion e*Q the incident ray. Since this is everywhere parallel 

 to the axis it must have a corresponding ray which will pass through the ante- 

 rior focus F. Prolonging the incident ray until it meets the refracted ray again, 

 we get the point e. 



The rays Px and Fx^ therefore, converge toward the point e, the rays Qy 

 and F*y 19 toward e* i. e., if we regard e as a luminous point e* will be its image. 



