VI 



DIOPTKIC MECHANISM OF THE EYE 



293 



linage 



rr is the emmetropic position of the retina ; r'r' the 

 hypermetropic position ; r" r" the myopic position, relatively to 

 the position of the two focal points and /. With the wide pupil 

 ab there are wide diffusion-circles a'b' and a"b" ; with the narrow 

 pupil cd the circles are less in diameter, c'd', c"d". 



V. We have seen that the emmetropic eye at rest has its 

 posterior focal point in the sensitive outer layer of the retina, and 

 is therefore able to see very distant objects. But in order to see 

 distinctly near objects which are projected in a plane behind 

 the retina the emmetrope must be able to increase the refraction 

 of his eye by a proportional increase in the curvature of the lens. 

 This active increase of the refractive power of the lens so as to 

 adapt it to the distinct vision of near objects is the special function 

 of the muscular mechanism that is associated with the dioptric 

 apparatus of the eye and is known as accommodation. To 

 distinguish between the clear emmetropic vision of distant objects 

 in repose without active [intervention of the mechanism of 



fia. 131. Diagram to show how the diffusion-circles alter when the pupil is contracted. 



accommodation and the distinct vision of near objects in which 

 such active intervention is required, the former is termed vision 

 by static refraction, the latter vision by dynamic refraction. 



The degree of adaptation, i.e. of active increase in the curvature 

 of the lens, rises gradually with the nearness of the fixed object 

 to the eye. The following table gives the position of the image 

 with different distances of the object, calculated for the reduced 

 eye:- 



Accordingly when the object is at a distance of 5 m. from the 

 reduced eye the image is only moved back O'OG mm. 



As the sensitive layer of the retina is just about that thickness 

 (O'OG mm.) it follows that at a distance of 5 m. the emmetropic 



