316 THE SPECIAL SENSES. 



abnormality in the refractive surfaces or the shape of the eyeball 

 prevents this exact focusing of parallel rays and makes the eye 

 ametropic. The most common refractive troubles of the eye 

 are due to short-sightedness or myopia, far-sightedness or hyper- 

 metropia, astigmatism, and old-sightedness or presbyopia. Some 

 description of these conditions is useful to emphasize by contrast 

 the mode of action of the dioptric mechanism in the normal eye, 

 but for a full description of the extent and complexity of these 

 defects reference must be made to special treatises upon the 

 errors of refraction in the eye. 



In myopia or near-sightedness parallel rays of light are brought 

 to a focus before reaching the retina. Consequently when the rays 

 fall upon the retina each point forms a diffusion circle and the image 

 is indistinct. This defect may be due to an abnormally great cur- 

 vature of the refractive surfaces, the cornea or the lens, or to an ab- 

 normal length of the eyeball in its anteroposterior diameter. The 

 latter cause is the more common. The defect may be congenital, 

 but usually it is acquired, and in the latter case its cause is generally 

 attributed to a weakness in the coats of the eyeball. The interior 

 of the eye is under some pressure, intra-ocular tension, which is 

 estimated to be equal to the pressure of a column of mercury 25 to 30 

 mms. in height. This tension is increased by strong convergence of 

 the eyeballs in looking at near objects. If the coats of the eye are 

 weak or become so from disease or malnutrition they may yield 

 somewhat to this pressure and the eyeball become lengthened in the 

 anteroposterior diameter. The condition as regards refraction of 

 parallel rays is represented then by the diagram B, in Fig. 132. 

 The retina is farther. away than the principal focal distance of the 

 refractive surfaces, and if the defect is excessive even diverging 

 rays may not be focused. The obvious remedy for such a condition 

 is to use concave lenses before the eyes for distant vision. By this 

 means, if the lenses are properly chosen, the rays will be given such 

 an amount of divergence that the focus will be thrown back to the 

 retina. As compared with the normal or emmetropic eye, the 

 myopic eye has its far point of distinct vision — that is, the farthest 

 point that can be seen distinctly without an effort of accommo- 

 tion — less than twenty feet from the eye, the exact distance depend- 

 ing upon the extent of the myopia. On the contrary, the near point 

 of distinct vision — that is, the nearest point at which distinct vision 

 can be obtained with the aid of the muscles of accommodation — is 

 closer than in the normal eye. Much of the prevalent myopia in the 

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

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

 or the head. Such conditions lead to an excessive muscular effort 

 and thus aggravate any tendency that exists toward the develop- 

 ment of a near-sighted condition. 



