260 HUMAN PHYSIOLOGY. 



6. It can also be seen from Fig. 27 that the size of the object is 

 to the size of the image as the distance of the object from the nodal 

 point K is to the distance of the image from K. 



7. An optical system may contain several spherical surfaces separat- 

 ing several refracting media. If all the centres of a spherical surface 

 lie in a straight line, the system is called a centred system, and the 

 straight line in which all the centres are located is called the optical 

 axis. The refraction of such a system can be determined by find- 

 ing the refraction of each surface successively according to the above 

 formulae. 



8. A system in which the entering rays are converged is called a 

 converging or collecting system [Sammelsystem] . (Parallel rays 

 are converged ; convergent rays are rendered more convergent ; 

 divergent rays are either rendered less divergent, parallel, or con- 

 vergent, according to the original degree of divergence. ) 



I. The dioptric system of the normal resting eye. The 

 dioptric system of the eyes is a convergent system of three 

 approximately concentric spherical surfaces placed between 

 four media. The media are: air, aqueous humor,- lens, 

 vitreous humor. The surfaces of separation are the anterior 

 surface of the cornea and the anterior and posterior surfaces 

 of the lens. The optical axis is called the visual axis (see 

 Fig. 28, /, f.J. 



The posterior surface of the cornea is disregarded because it is 

 parallel with the anterior surface and because the index of refrac- 

 tion of the cornea may be regarded as the same as that of the 

 aqueous humor. 



The indices of refraction of the aqueous humor and of the 

 vitreous humor are 1.338, that of the lens is 1.455. The 

 radius of the curvature of the corneal surface is 8 mm, of the 

 anterior surface of the lens 10 mm, of the posterior surface 

 of the lens 6 mm. The distance of the anterior surface of 

 the cornea from the anterior surface of the lens is 3.6 mm, 

 the thickness of the lens is also 3.6 mm. The retina lies 15 

 mm behind the posterior surface of the lens. From these 

 data the dioptric action of the system can be found. 



The indices of refraction can be determined only in the 

 dead eye, but the radii and the distances of the surfaces can 

 also be determined in the living eye. 



The lens is composed of many layers, like an onion, and the 



