cfi. LVIIL] 



ERRORS OF REFRACTION 



829 



etc. They rest the eye by relieving the ciliary muscle from excessive 

 work. 



FIG. 529. Diagram showing 1, normal (emmetropic") eye bringing parallel rays exactly to a focus on 

 the retina ; 2, normal eye adapted to a near-point ; without accommodation the rays would be 

 focussed behind the retina, but by increasing the curvature of the anterior surface of the lens 

 (shown by a dotted line) the rays are focussed on the retina (as indicated by the meeting of the two 

 dotted lines) ; 3, hypermetropic eye ; in this case the axis of the eye is shorter than normal ; parallel 

 rays are focussed behind the retina ; 4, myopic eye ; in this case the axis of the eye is abnormally 

 long ; parallel rays are focussed in front of the retina. The figure incorrectly represents the 

 refraction as occurring only in the crystalline lens ; the principal refraction really occurs at the 

 anterior surface of the cornea. 



3. Astigmatism. This defect, which was first discovered by 

 Airy, is due to a greater curvature of the eye in one meridian than 

 in others. The eye may be even myopic in one plane, and hyper- 

 metropic in others. Thus vertical and horizontal lines crossing each 

 other cannot both be focussed at once; one set stand out clearly, 

 and the others are blurred and indistinct. This defect, which is 

 present in a slight degree in all eyes, is generally seated in 

 the cornea, but occasionally in the lens as well; it may be 



