HYGIENE OF THE SENSE ORGANS 



237 



retina from the lens. These errors of refraction can be cor- 

 rected by suitable glasses or spectacles (see Fig. 118). As age 

 advances the lens in many persons loses its elasticity, so that 



Fig. 118. Eyes and spectacles 



A "normar' eye, one in which the curvature of the lens matches perfectly the length 

 of the eyeball, a, is very rare. If the curvature of the lens is relatively too great 

 the image falls in front of the retina, b; this near-sightedness can be made up by a 

 concave lens of suitable curvature. Most eyes (of human beings) are far-sighted, so 

 that the image of near objects falls behind the retina, c; a convex lens compensates 

 for the flatness of the eye lens. Sometimes far-sightedness is due to extreme short- 

 ness of the eyeball, d: a convex lens is then necessary to bring the image forward 

 to the retina, c. The opposite defect, extreme length of eyeball, /, is compensated 

 by a concave lens, g. Spectacles do not correct defects of the eye; they can only 

 correct the vision by compensating or making up for certain defects. Thus they can 

 improve vision and reduce strains. (The curvatures and proportions are exaggerated 



in these figures) 



it is necessary for those who do close work to have one pair of 

 glasses for far vision and another for reading, sewing, and so on. 

 The lens surface of some eyes is uneven, so that the rays com- 

 ing in are not turned equally from all parts of the field. This 



