SENSE ORGANS AND SENSATIONS 249 



near objects the ciliary muscle must work much harder 

 than in the normal eye, and it often happens that, even 

 with its utmost effort, the rays are not sufficiently bent to 

 focus them on the retina ; so that a book, for example, must 

 be held at arm's length to be read. Persons having such 

 eyes form one class of those said to be " far-sighted," and 

 their trouble can be corrected by the use of convex glasses. 



As old age ap- 

 proaches, changes oc- 

 cur in the lens; in 

 consequence, it no 

 longer becomes as 

 convex as formerly 

 in response to the ac- 

 tion of the muscle 

 of accommodation 

 (presbyopia, from 

 irplffflvt, " old "). 

 Some, though not 

 all, results of this 

 condition resemble 



those of hyperme- 



i , i FIG. 98. A test for astigmatism 



tropia; but the two 



differ in cause. Hypermetropia is due to shortness of eyeball ; 

 presbyopia, to failure of accommodation. 



11. Astigmatism. We have thus far been dealing with 

 those optical imperfections due to improper distance between 

 the lens and the retina. Another and frequently more seri- 

 ous trouble, known as astigmatism, results when the cur- 

 vature of the cornea (and sometimes of the lens) is not 

 perfectly regular; that is, when these surfaces are not seg- 

 ments of perfect spheres, but resemble in curvature the 

 side of a lemon. In this case the rays of light from a point 

 are not brought to a focus again in a point behind the 

 lens; and remembering the importance of sharp focusing in 



