SCHOOL HYGIENE 437 



and the retina which lines the eye corresponds to the sensi- 

 tive surface on the film or glass on which the picture is taken. 



In the average good eye, rays of light pass through the 

 eye lens and are focused on the retina, where an inverted 

 image is formed in each eye. This causes impulses to pass 

 along nerves to the brain, which give us the sensation of a 

 single image right-side up. The brain inverts the images 

 and causes us to see one image instead of two. 



Kinds of defects. There are three common defects of 

 the eye: nearsightedness, farsightedness, and astigmatism. 

 In nearsightedness, the rays 

 af light come to a focus be- 

 fore they reach the retina, 

 and hence the image is 

 olurred. In farsightedness, 

 the rays would focus behind 

 she retina if they could pass 

 through this. In astigma- 

 tism, the curvature of the 

 lens is not the same in all 



., FIG. 173. Section of the human eye. 



directions and while a ver- 

 tical line may be clearly focused, a horizontal line is blurred. 

 The eye attempts to remedy these defects by accommoda- 

 ;ion; but this involves a straining of the eye muscles, es- 

 pecially in farsightedness and astigmatism, and if carried 

 too far, results in eye ache and headache. 



Remedies. The remedy for nearsight is a concave lens. 

 This helps to throw the focus further back on the retina. 

 The remedy for farsight is a convex lens. This brings the 

 light to a focus more quickly. The remedy for astigmatism 

 is a lens with unequal curvature in different directions to 

 offset the curvature of the eye lens. 



The science of the oculist has developed so far that most 

 /3f these common defects can be easily remedied by the use 

 of spectacles. In the case of a very nearsighted person, a 



