THE SPECIAL SENSES 291 



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 further 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. (Read Note 11.) 



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 prevalent fallacies 

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

 quote from the highest authority, Prof. Bonders, 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. 

 Some have thought, by the early use of spectacles, to be able to preserve 

 their power of vision, and have recommended and employed ' conservative 

 glasses.' If I am not mistaken, self-interest had something to do with 

 this recommendation. So long as the eye does not err, and remains free 

 from fatigue in the work required of it, its own power is sufficient, and it 

 is inexpedient to seek assistance in the use of convex glasses. Having 

 made up their minds that they require glasses, how are they to know what 

 glasses to' procure, or what number is correct ? Generally, people go 

 to the nearest spectacle-vender, and purchase what they see best with at 

 the time. We say distinctly, once for all, that the ophthalmic surgeon is 

 the one to be consulted as to the wearing of glasses. He, by testing the 

 eye, can alone decide whether any, and what glasses, should be worn. 

 Opticians and spectacle-venders know nothing about the laws which 

 govern the refraction and accommodation of the eye. It is not their 



84. Change of sight with the approach of old age 5* Explain the change. 



