328 THE HUMAN BODY. 



to a focus behind the retina ( C). To see even distant ob- 

 jects, such persons must therefore use muscular effort to 

 increase the converging power of the lens; and when objects 

 are near they cannot, with the greatest effort, bring the 

 rays proceeding from them to a focus soon enough. To 

 get distinct retinal images of near objects, they therefore 

 need converging (convex) spectacles. Such e}'es are called 

 hypermetropic, or in common language long-lighted. 



Hygiene of the Eyes. The healthy eye is so constructed 

 that when its muscles are at rest distinct images of distant 

 objects are focussed on the retina. To see near objects 

 muscular effort is required; hence the greater fatigue which 

 follows long gazing at them. 



In a hyperme tropic eye more effort is needed to see near 

 objects, and this results in muscular fatigue. Hyper- 

 metropic persons can often read well for a while, but then 

 complain that they can no longer see distinctly. This kind 

 of weak sight should always lead to examination of the 

 e} r es by an oculist, to see if glasses are needed; otherwise 

 severe neuralgic pains about the eyes are apt to come on, 

 and the overstrained organ may be permanently injured. 



Children sometimes have hypermetropic eyes, and in 

 that case should be at once provided with suitable spec- 

 tacles. In old age another kind of long-sightedness (pres- 

 byopia) is common: it is due to too great stiffness of the 

 crystalline lens, which does not become convex enough 

 during accommodation to focus on the retina the images of 

 near objects. 



Short-sighted eyes appear to be more common 

 now than formerly, especially in those given to indoor 



Why is the eye apt to be fatigued by the continued contempla- 

 tion of near objects ? 



How does the hypermetropic eye differ from the normal in the 

 above respect? What should be done at once if a child is found to 

 have hypermetropic eyes? What is presbyopia? 



In what classes of persons is myopia most frequent? 



