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PHYSIOLOGY FOR DENTAL STl'DENTS. 



short-sightedness or myopia, and astigmatism (Fig. .">."> |. /////>, /- 

 i< Iropia is due to the eyeball being too short so that the focus 

 of the image is behind the retina. The error is corrected by 

 prescribing convex glasses. Myopia is due to the opposite con- 



Fig. 55. Errors in refraction : E shows the formation of the image on 

 the retina in the normal or emmetropic eye ; H shows the condition in long- 

 sight, or hypermetropia, where the eyeball is too short; M shows the condi- 

 tion in short-sight, or myopia, where the eyeball is too long. 



dition, that is, the eyeball is too long, so that the focus occurs 

 in front of it. Concave glasses correct it. AstiyvKttixm is due to 

 the lens or cornea being of unequal curvature in its different 



