238 BIOLOGY AND HUMAN LIFE 



condition, which is called astigmatism, may cause severe strain 

 and headaches, especially with those who have to discriminate 

 views or patterns in which lines are important, as in reading. 

 It cannot be corrected, but can be compensated by suitable 

 spectacles that are ground with a cylindrical surface. 



Another kind of strain is the effect on the eyeballs of unequal 

 muscular arrangement, which causes the axis of one eye to turn 

 too far inward or outward. Strabismus, or squint, can be rem- 

 edied by a simple surgical operation. In some cases the use of 

 special wedge-shaped or prismatic glasses is sufficient. 



If the eyes have to be examined, it is well to go to a com- 

 petent eye doctor, or oculist, rather than to an optician. A 

 scientific examination of the eyes sometimes reveals serious con- 

 ditions. The optician is trained to discover shortcomings in re- 

 fraction, but he is not able to discover other sick conditions. 

 The spectacles which he prescribes may be suitable for what 

 he finds, but they cannot help the other conditions. 



Since the eye is particularly sensitive to light, incorrect illu- 

 mination may affect the eye unfavorably, and through the eye 

 the general health. Prolonged exposure of the eye to light will 

 cause fatigue, and in extreme cases pain, discomfort, or even 

 temporary blindness. For our daily routine we should therefore 

 avoid work in direct sunlight, where the light can be reflected 

 directly into the eyes. Flickering lights and sudden flashes 

 cause injurious strains, since the iris cannot move fast enough 

 to protect the retina from excessive exposure, and the iris reflex 

 may become overworked. Another source of injury to the retina 

 is glare, which is produced when a strong light strikes the retina 

 while the pupil is open, or when a strong light strikes part of 

 the retina while the rest is in comparative darkness. 



The eyeball, in its bony setting, is fairly well protected from 

 injury by large bodies, and the very quick eyelash reflex keeps 

 many small particles out. Nevertheless many eyes are injured 

 every year either by blows or by dust. In railroading, in the 

 building trades, and in other dusty occupations, cinders and 

 flying particles of metal, stone, brick, coal, etc. are sources of 



