THE OPTICAL SYSTEM OF THE EYE 



523 



is still parallel to the path which it travelled initially. A ray passing through 

 either focus leaves the lens system on the other side parallel to the axis. It 

 is further found that the distances between some of these points are equal, 

 for example :(/>h - K'<' and W = <K. Also hh' == KK'. The 

 positions of these cardinal points has been determined in the case of the 

 eye with considerable accuracy ; the following approximate values may be 

 given : 



Distance from front of cornea to first principal point h . 1-7 mm. 



to second principal point h' . 2-0 mm. 



to first nodal point K . 7 -Omm. 



to second nodal point K' . 7-3 mm. 



to anterior focus </> .13*6 mm. 



to posterior focus </>' . 22 '6 mm. 



to retina . . . . . 22 '6 mm. 



The position of these points being determined, the direction of the rays 

 of light through the eye can be easily obtained, and is shown dia grammatically 

 in Fig. 263. 



FIG. 263. The optical system of the eye shown diagrammatically. 

 H and H' principal points. K and K' nodal points. <f> and $' anterior and 

 posterior foci. 



It is of some interest to know the relative part taken by the various 

 refractive media of the eye in the formation of the image. By far the 

 greater part is performed by the cornea. The following values in mm. 

 and dioptres may be given : 



Focal length of the cornea . 

 of the lens 

 of the whole eye 



w 



remo^ 



Mm. 



. 24 

 . 44 

 . 15-5 



Dioptres. 



42 

 23 

 65 



hen opacities form in the lens (cataract) this structure is 

 oved by operation. It is then found that a lens of approximately 

 23 dioptres has to be worn by the patient in order that he may 

 see distinctly. When the eye is placed under water the refraction of the 

 cornea is necessarily abolished because water has approximately the same 

 refractive index. Under these conditions the eye becomes too long-sighted 

 for distinct vision. The eye of the fish has met this difficulty by the provision 



