REACTION TO LIGHT 315 



The contraction of the pupil for near objects is not, properly 

 speaking, a part of accommodation. 



Then, granting special sensibility to the retina and optic nerve 

 the formation and appreciation of an image is simple. Rays of 

 light having passed through the cornea and aqueous humor are 

 admitted by the pupil to pass through the lens and vitreous hu- 

 mor. By all these objects they are refracted so that they cross 

 each other and fall upon the retina, producing an inverted image 

 there. The size of the pupil, other things being equal, is regu- 

 lated by the intensity of the light, the opening being contracted to 

 admit less when the light is strong. 



Myopia, Hyperopia and Presbyopia. Sometimes the antero- 

 posterior diameter of the eye-ball is too long and the rays of 

 light are brought to a focus in front of the retina. Such a con- 

 dition is known as myopia; the person will be near-sighted. 

 He brings objects near his eyes so that the rays may have a greater 

 divergence and thus be focused farther back. Or the rays may 

 be scattered by placing concave lenses before the eyes. Some- 

 times, too, the antero-posterior diameter may be too short and 

 the rays come to a focus behind the retina. Such a condition is 

 known as hyperopia ; the person will be far-sighted. He holds 

 objects far away from his eyes that the rays from them may strike 

 the ball with less divergence and thus be focused farther forward. 

 Or the same end may be accomplished by placing convex lenses 

 before the eyes. In old age the lens becomes flattened and accom- 

 modates itself less easily. This tends to focus light behind the 

 retina and objects have to be held far away from the eye. This 

 is known as presbyopia. Its remedy is the same as that for 

 hyperopia. 



Reaction to Light. Regarding the reaction of the pupil to 

 light, it is evident that this is mainly a reflex nervous phenom- 

 enon, though direct light will cause the muscular tissue of the 

 iris to contract. The direct influence of the third nerve on the 

 action of the iris has been referred to under a consideration of 



