236 ELEMENTARY BIOLOGY 



upon the general health. As light acts in the eye by bringing 

 about chemical changes in certain cells of the retina (the 

 "rods and cones"), prolonged exposure to light will carry 

 these changes so far that it is no longer possible to see. 

 Even before this extreme condition is reached, the cells will 

 show signs of fatigue, which may be accompanied by pain, 

 or at least by discomfort. This is what happens when the 

 eyes are long exposed to strong light, and the only way to 

 counteract the fatigue is by giving the eyes a complete rest. 

 Staying in a dark room or bandaging the eyes for from 

 twenty minutes to an hour will usually give the retinal cells 

 time to recover. 



Flickers and flashes. The iris opens or closes in accordance 

 with the intensity of illumination. Sudden increases in the 

 amount of intensity of the light reaching the retina is likely 

 to cause injury to the pigment cells. This is why flashes or 

 a flickering light will fatigue and strain the eye, and such 

 sources of injury should be avoided. 



Glare. A glare is produced when a comparatively strong 

 light strikes the retina while the pupil is open, or when 

 a strong light strikes a portion of the retina, the rest being 

 in comparative darkness. This condition results in injury to 

 the eye, and should be avoided as much as possible. 



279. Mechanical injury to the eyes. Although the eyeball, 

 in its bony setting, is fairly well protected against injury by 

 large bodies, and although the very quick eyelash reflex keeps 

 many small particles out, many eyes are injured every year 

 either by blows or by dust. In railroading, in the building 

 trades, and in other dusty occupations flying particles of stone, 

 metal, cinders, coal, brick, etc. are sources of serious danger 

 to the eyes of workers. Wherever possible, workers in such 

 occupations should wear goggles. In any case we must be care- 

 ful not to rub the eye when something has got under the lid, 

 and whoever tries to remove a particle from under the eyelid 

 must approach the task with perfectly clean hands. 



