296 



THE SPECIAL SENSES. 



young is attributed by oculists to bad methods in reading, such as 

 insufficient lighting, small print, and a faulty position of the book. 

 Such conditions lead to an excessive muscular effort and thus 

 aggravate any tendency that exists toward the development of a 

 near-sighted condition. 



In hypermetropia the conditions are the opposite of those in 

 myopia. Parallel rays of light after refraction in the eye cut the 

 retina before they come to a focus. The principal focal distance, in 



other words, is behind the 

 retina. In this case, also, 

 each point of a distant 

 object will make upon the 

 retina, when the eye is 

 not accommodated, a dif- 

 fusion circle, and the image 

 consequently is blurred. 

 This defect may be caused 

 by a lessened curvature or 

 refractive power in the 

 cornea or lens, but in the 

 majority of cases it is ref- 

 erable to a diminution in 

 the anteroposterior diam- 

 eter of the eyeball. This 

 condition is usually con- 

 genital: the eyeball from 

 birth is smaller than the 

 normal. The path of the 

 parallel rays in this case 

 is represented in the dia- 

 gram C, Fig. 125. When 

 such an eye looks at a dis- 

 tant object a clear image 

 may be obtained only by 

 using the ciliary muscle, 

 and to prevent this constant strain upon the muscle of accommo- 

 dation convex glasses must be worn. Glasses of this kind converge 

 the rays and if properly chosen will bring parallel rays to a focus 

 without the constant aid of accommodation. It is obvious that in 

 the hypermetropic eye there is no far point of distinct vision when 

 the eye is at rest, since some accommodation must be used to bring 

 even parallel rays to a focus. The near point of distinct vision will 

 be farther away than in the normal eye, since accommodation be- 

 gins when the rays are parallel and its limits are reached with 

 a less degree of divergence; hence the name of far-sightedness. 



Fig. 125. Diagram showing the difference be- 

 tween normal (;1), myopic (B), and hypermetropic 

 (C) eyes. In B and C the dotted lines represent the 

 path of the rays after correction by glasses. (Bow- 

 ditch.) 



