vi DIOPTKIC MECHANISM OF THE EYE 291 



When his eye is at rest the myope cannot see distant objects 

 distinctly, and the hypermetrope cannot see near objects distinctly, 

 because in these opposite forms of ametropia diffusion-circles 

 instead of images are formed on the plane of the sensitive retina, 

 corresponding to the respective points of the object. Diffusion- 

 circles are circular surfaces of illumination, which result from the 

 transverse section of the cones of rays that enter by the pupil ; 

 when reciprocally superposed, these make the retinal image con- 

 fused and indistinct. 



This is clear from the diagram (Fig. 130), in which the states 



C 



FIG. 130. Diagram to show the static refraction of the eye. (After Cohn.) A, hypermetropic ; 

 B, emmetropic ; (7, myopic eye ; a., optic axis ; ., nodal point at which the parallel rays that 

 enter the eye converge ; c., diffusion-circles. 



of refraction of three types of eyes, resulting from different lengths 

 of the axis, are compared. 



The far point of the eye is that which forms a sharp image 

 upon the sensitive layer of the retina when the eye is at rest. In 

 the emmetropic eye it is at infinite distance ; in the myopic eye 

 objects at a definite distance are focussed in front of the retina ; 

 in the hypermetropic eye at a short distance behind. Accordingly, 

 in myopia the relative refractive power of the eye is in excess, in 

 hypermetropia it is too low. 



The degree of myopia or of hypermetropia is determined 

 accurately by the refractive power of the lenses (concave or convex) 

 required to make the myopic or hypermetropic eye emmetropic 

 in rest. 



