SENSE OF SIGHT. 561 



object passes to reach the retina were all of the same density as the 

 air, and were also plane surfaces, an impression would be produced, 

 but there would be no distinct image. Actually, before such rays 

 do reach the retina they pass through certain media which, by 

 reason of both density and shape, refract them and bring them to 

 a focus, thus producing a sharp and distinct image of the object 

 looked at. These media are the cornea covered with a layer of 

 tears, aqueous humor, crystalline lens with its anterior and poste- 

 rior capsule, and vitreous humor, seven in all. The amount of 

 refraction is determined by the radius of curvature of the surface 

 through which the rays pass, the refraction being greater as the 

 radius becomes smaller, and by the difference between the refrac- 

 tive indices of the media, refraction increasing as the difference 

 increases. The radii of curvature are as follows : 



When accommodated for 



Far vision. Near vision. 



Cornea 8mm. 8 mm. 



Anterior surface of lens 10 " 6 " 



Posterior " " " 6 " 5.5 " 



The refractive indices of the various media through which the 

 light passes are as follows : 



Tears l.J 



Cornea 1.337 



Aqueous humor 1.3365 



Vitreous " 1.3365 



Lens (mean for all layers) 1.437 



The refractive index of air is 1.000, and of water 1.335. From 

 this table it will be seen that the tears, cornea, and aqueous humor 

 have practically the same indices of refraction, and we may there- 

 fore regard the media through which light passes to reach the 

 retina as three in number: 1, tears, cornea, and aqueous humor, 

 with a refractive index of 1.33; 2, crystalline lens, with a re- 

 fractive index of 1.43; and 3, vitreous humor, with a refractive 

 index of 1.33. 



The following table gives the distances between the various 

 points mentioned therein : 



When accommodated for 



Far vision. Near vision. 



Anterior surface of cornea and anterior surface of lens . . 3.6 mm. 3.2 mm. 



" " " " posterior " " . . 7.2 " 7.2 " 



" lens " " " " . . 3.6 " 



Posterior " " retina " . . 14.6 " 14.6 " 



The anteroposterior diameter of an em me tropic eye along the 

 axis is 21.8 mm. 



36 



