THE SPECIAL SENSES 



341 



called accommodation ; it is brought about by means of 

 the muscular fibers around the lens. Straining of the 

 muscles is required for looking at very near objects. 



616. Defects of Vision. In near-sighted eyes (Fig. 218) 

 the eye is too long from front to back, and the rays come 

 to a focus before reaching the retina. Among savage na- 

 tions, where no books are used, almost every one has good 

 sight, and nearsightedness is hardly known. Nearsighted- 

 ness may be inherited or may begin with children at school. 

 Some children seem to have a natural wish to get their 

 eyes close to the book or writing. This is the very worst 



FIG. 218. 



Myopic, or Nearsighted, Eye, in 

 which Parallel Rays of Light 

 (A", A"} focus the Image in Front 

 of the Retina at C' , producing a 

 Blurred Image on the Retina at 

 B ff , the Rays diverging from C 1 . 



FIG. 219. 



Hypermetropic, or Farsighted, Eye, in 

 which Parallel Rays of Light (A' t 

 A f ) are focused behind the Retina 

 at C, producing a Blurred Image 

 at B>. 



thing for nearsightedness. The head should be held erect 

 in reading, to prevent the blood congesting in the eyes, 

 and to prevent round shoulders and flat chest. The proper 

 distance for reading is fourteen to eighteen inches. The far- 

 sightedness (Fig. 2 1 9) that occurs in youth is caused by the 

 eyeball being too flat. In the farsightedness of old age the 

 lens has also lost its elasticity, so that its shape cannot be 

 sufficiently changed to bring the lights from near objects 

 to a focus on the retina. In farsightedness, convex glasses 

 are used, and in nearsightedness, concave glasses are used. 

 Astigmatism is a defect caused by unequal curvature of the 

 cornea in different directions (Fig. 220). 



617. Regulation by Iris of Amount of Light admitted. 

 Look toward a bright window or the sky and note 



