DIOPTRIC MECHANISMS OF THE EYEBALL 533 



(3) The radius of curvature of the anterior surface of the lens. 



(4) The thickness of the lens. 



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



In addition we must know the refractive index of the cornea and 

 aqueous or vitreous humour, and also the average refractive index of the 

 lens. There is a considerable difference between the refractive indices of 

 the outer and the inner portions of the lens, the inner part being much 

 denser and having a higher refractive index than the periphery. The 

 following Table represents the constants of a human eye as determined by 

 Helmholtz : 



Refractive index of aqueous and vitreous humours . . 1-3365 



Total refractive index of lens ..... 1-4371 



mm. 



Radius of curvature of cornea .... g 



anterior surface of lens ... 10 



posterior surface of lens ... 6 



Distance from anterior surface of cornea to anterior surface of 



lens ... 3-6 

 posterior surface 



of lens . . 7-2 



Anterior focus of cornea ....... 23-692 



Posterior focus of cornea ....... 31-692 



Focus of lens ....... 43-707 



Posterior focus of eye ....... 19-875 



Anterior focus of eye ........ 14-858 



Distance from anterior surface of cornea to : 



First principal point ...... 1-9403 



Second principal point ..... 2-3565 



First nodal point 6-957 



Second nodal point ...... 7-373 



Anterior focus of eye ...... 12-918 



Posterior focus of eye 22-231 



It will be seen that both the principal points lie in the anterior chamber, 

 while the nodal points fall in the back part of the lens. The posterior focus 

 of the eye falls upon the retina. For many purposes we may simplify our 

 calculations by running the two principal points and the two nodal points into 

 one. In such a reduced eye the single principal point is situated 2-3 mm. behind 

 the anterior surface of the cornea, and the single nodal point 047 mm. in 

 front of the hinder surface of the lens. If a circle be drawn from the single 

 nodal point as a centre through the single principal point as a circumference 

 we get a surface II in the figure, which represents the anterior refracting 

 surface of such a reduced eye. 



A reference to the Table of Constants of the human eye shows that, 

 whereas the anterior focal distance of the cornea is 23 mm., that of the whole 

 eye is 15 mm. and that of the lens 44 mm. It is evident from this that the 

 anterior surface of the cornea is the most important refractive surface of the 

 eye, and that, in the convergence of rays necessary for the formation of an 

 image on the retina, it is the refraction at this surface which plays the 



