1 1 4 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



inches, according to the degree of myopy. The fault 

 being too great refraction, the remedy is, of course, to 

 diminish the refraction by the use of concave glasses. 

 If these be so chosen that the focus of parallel rays is 

 on the retina when the eye is passive, so that distant 

 objects are seen distinctly; then the eye accommodates 

 itself to all nearer objects, and behaves in all respects 

 like a normal eye. 



Hyperopy— Oversightedness.— This is the oppo- 

 site of myopy. In the passive state the focus of parallel 

 rays is behind the retina. In this defect the refractive 

 power of the lenses is too small for the position of the 

 retina. The defect is far more common than generally 

 supposed. It often exists unknown to the patient or his 

 friends. Distant objects are seen perfectly well, because 

 a slight accommodation brings the focus on the retina. 

 But the eye is never passive unless in sleep. For this rea- 

 son it is often a distressing defect, producing headaches 

 and the like. Since the defect is a deficiency in refrac- 

 tive power, the obvious remedy is the use of slightly 

 convex glasses suited to the degree of deficiency. The 

 eye then functions exactly like a normal eye. 



Presbyopy— Old-sightedness. — Both the preced- 

 ing are strnctuial defects; this is a functional defect. 

 The eve may be structurally normal — i. e., in a passive 

 state the focus of parallel rays is on the retina — but it 

 has lost the power of accommodating itself to divergent 

 rays. The patient sees well distant objects, but can not 

 see near objects well. In order to see near objects well 

 the eye must be re-enforced by convex glasses. But 

 the use cf glasses can not make the eye normal, as in 

 the other two defects, because it has lost the accommo- 

 dating power. Therefore the glasses are not worn ha- 

 bitually, as in the other two defects, but only in looking 

 at near objects — not in walking, but only in reading. 



