738 



THE SENSES 



on the retina. Thus, the images of distant objects are blurred and in- 

 distinct. The eye is, as it were, permanently adjusted for a near point. 

 Rays from a point near the eye are exactly focused on the retina. But 

 those which issue from any object beyond a slight distance, the myopic 

 far-point, which is less than twenty feet, cannot be distinctly focused. 

 This defect is corrected by concave glasses, which cause parallel rays to 

 diverge before entering the eye. Such glasses of course are needed only 

 to give a clear vision of distant objects. For near objects they are not 

 required. 



FIG. 466. Diagram Showing: i, Normal or emmetropic eye bringing parallel rays 

 exactly to a focus on the retina; 2, normal eye at rest, showing that light from a near point is 

 focused behind the retina, but by increasing the curvature of the anterior surface of the 

 lens (shown by dotted lines) the rays are focused on the retina; 3, hypermetropic eye. In 

 this case the axis of the eye is shorter and the lens normal (or the lens may be flatter than 

 normal and the eyeball normal) ; parallel rays are focused behind the retina; 4, myopic eye. 

 In this case the lens is too convex (or the axis of the eye is abnormally long); parallel rays 

 are focused in front of the retina. 



Hypermetropia. This is that refractive condition of the resting eye 

 in which parallel rays are still converging but not yet focused at the retina, 

 are brought to a focus behind the retina, 3, figure 466. It is the opposite 



