592 PHYSIOLOGY 



human eye. The rays of light pass through the following media, 

 cornea^ aqueous humour, lens, and vitreous humour, and may be 

 refracted at the following surfaces ; anterior and posterior surfaces of 

 the cornea, anterior and posterior surfaces of the lens. Since, however, 

 the refractive indices of cornea, aqueous humour, and vitreous humour 

 are practically identical, we can reduce the refractive media to two, 

 viz. cornea, aqueous, and vitreous in the one case, and lens in the other ; 

 and the refractive surfaces to three, viz. anterior surface of cornea, 

 anterior surface of lens, and posterior surface of lens. 



In order to determine the path of the rays in the eye we have to 

 determine the following data. viz. : 



(1) The radius of curvature of the anterior surface of the cornea. 



(2) The distance of the anterior surface of the cornea from the 

 anterior surface of the lens. 



(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 refraction index of lens ...... 1-4371 



mm. 

 Radius of curvature of cornea ...... 8 



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 



* Quoted by Tigerstedt. 



