Light 



189 



DIAGRAM OF THE EYE IN NEAR SIGHT. THE DEFECT Is REMEDIED BY 

 CONCAVE GLASSES 



The lens b brings the rays from a point of light a together at c too soon. So the rays 

 cross and fall over the whole surface of the retina from d to e making a confused 

 image instead of a clear point. When the rays are spread apart by bringing the light 

 near the eye, as at f, they come together farther away upon the other side of the 

 lens, as at g. Thus they fall upon a single point of the retina and produce a clear image. 



DIAGRAM OF THE EYE IN FAR SIGHT. THE DEFECT Is REMEDIED BY 

 CONVEX GLASSES 



The lens b does not bring the rays from a point of light a together soon enough. 

 So the rays fall over the whole surface of the retina from d to e, making a confused 

 image instead of a clear point. When the rays spread less apart, as when the light is 

 moved farther away, to /, the lens brings them together sooner. Then the rays fall 

 upon a single point of the retina at g, and thus form a clear image. 



Astigmatism. Astigmatism is a defect in the eye due to 

 irregular curvature of the cornea or of the lens, making it difficult 

 to see equally well lines running in different directions. Such a 

 defect causes strain of the eye muscles in changing the focus so 

 as to try to get a clear, distinct image. The effort often causes 

 headache and nervousness. Glasses are required to remedy this 

 defect. 



Precautions for the care of the eyes. 



1. Avoid using the eyes where there is insufficient or un- 

 steady light. 



