226 



THE EYE 



It is only in the emmetropic eye that convergence and accom- 

 modation correspond exactly in amount. A hypermetropic 

 person viewing a near object may require to use, say 4 dioptres 

 of accommodation, whereas an emmetropic person uses only 

 2 dioptres, but both employ the same amount of convergence. 

 Similarly an emmetrope may focus a near object, and if concave 

 lenses are placed in front of his eyes he can continue to see the 

 object plainly by increasing his accommodation sufficiently to 

 neutralise their effect. The object still remains at the same dis- 

 tance, i.e. the convergence does not alter. Hence we see that 

 accommodation and convergence are to some extent independent. 



FIG. 52. Convergence. 



FIG. 53. Artificial divergence of visual axes. 



3. Divergence. In normal circumstances the visual axes never 

 diverge, for they are in parallel adjustment for objects at infinity. 

 Divergence can however be brought about artificially. Thus if 

 we interpose prisms to render the rays divergent we can produce 

 a corresponding divergence of the visual axes. 



4. Visual Judgments. On account of the distance between the 

 eyes, objects are viewed from a slightly different angle by each 

 eye. This is readily demonstrated by looking at an object first 

 with one eye and then with the other. It is particularly well 

 brought out by two objects almost or completely in alignment, 

 and the phenomenon persists at long distances. Each retina thus 



