392 



THE BODY AT WORK 



objects are focussed behind the retina. Concave glasses cor- 

 rect the one condition, convex glasses correct the other. 

 Glasses are also very commonly called for to neutralize another 

 defect regular astigmatism which may be present by itself, 



FIG. 33. 



A The normal eyeball, in which, when the ciliary muscle is relaxed, parallel rays are brought 

 to a focus on the retina. B, A hypermetropic eyeball. Its depth being less than normal, 

 parallel rays are not brought to a focus on the retina when the eye is adjusted for distant 

 vision without the aid of a convex glass. C, A myopic eyeball. Its depth being more 

 than normal, a concave lens is needed to diminish the convergence of parallel rays. 



or may accompany insufficient length or too great length of 

 the optic axis. It is due to unequal curvature of the cornea. 

 Usually the curvature is sharper in the vertical than in the 

 horizontal meridian (cf. p. 269) ; as a consequence, points in a 

 vertical line are focussed in front of points in a horizontal line. 



