304 THE SPECIAL SENSES 



84. Old-sight, or Presbyopia. But this marvellously beauti- 

 ful mechanism becomes worn with use ; or, more strictly speak- 

 ing, the lens, like other structures of the body, becomes harder 

 with the approach of old age. The material composing the 

 lens becomes less elastic, the power to increase its curvature is 

 gradually lost, and as a consequence, the person is obliged to 

 hold the book farther away when reading, and to seek a 

 stronger light. In a word, the function of accommodation 

 begins to fail, and is about the first evidence that marks the 

 decline of life. By looking at the last preceding diagram, and 

 remembering that the increased curvature of the lens cannot 

 take place, it will be at once understood why old-sight is bene- 

 fited in near vision by the convex lens, such as the spectacles 

 of old people contain. It acts as a substitute for the deficiency 

 of the crystalline lens. (Head Note 11.) 



85. Astigmatism. It often happens that the curvature of 

 the cornea or of the crystalline lens, or of both, is not uniform. 

 This is more often true of the cornea than of the crystalline 

 lens. When this happens, rays of light which pass through 

 that portion of the cornea (or lens) having a greater curvature, 

 will be refracted to a focus more quickly than the rest. Under 

 such circumstances the focus will not be a point, as it should 

 be for perfect vision, but a line. Such an eye is astigmatic. 

 This difficulty is very common, and should be corrected by 

 glasses fitted for each case. 



11. The Choice of Glasses. "The perfectly healthy, normal eye 

 begins to need a glass for ordinary work at between forty and forty-five 

 years of age of course, we here exclude all debilitated conditions of the 

 body resulting from disease. Now, then, comes the question shall we 

 put on glasses, and of what strength ?. To answer some prevalerit fallacies 

 handed down from one generation to another, we cannot do better than 

 quote from the highest authority, Prof. Donders, who says : ' The opinion 

 is rather general that we should refrain as long as possible from the use 

 of convex glasses. But, is it not folly to weary the eyes and the mind 

 together, without necessarily condemning ourselves to guess, with much 

 trouble, at the forms which we could see pretty well with glasses ? ' 



"Strangely enough, people have fallen also into the opposite fault. 



84. Change of sight with the approach of old age ? Explain the change. 



85. Astigmatism ? Its cure ? 



