CHAPTER XIX 



THE PRINCIPLES OF VISION 



Vision is dependent upon light. Rays of light from 

 a distance are parallel, while those from near objects are 

 divergent. 



Refraction} means the turning or bending of rays of 

 light as they pass through an object that is denser than 

 the air. A good illustration of this is the apparent bend- 

 ing of a spoon upward when placed in a glass of water, or 

 the displacement of an object when seen through a 

 prism. 



The index of refraction is the resistance of the object 

 through which the light passes as compared with air, 

 which is taken as i. 



When parallel rays of light pass through a plate-glass 

 with both surfaces parallel they are not refracted, but 

 emerge as they entered, but when they pass through a 

 glass that is thicker at one edge than the other (a prism), 

 they are deviated, or refracted from the apex toward the 

 base, or the thicker portion of the prism. The angle of 

 refraction — that formed by the incident ray with the 



^ Only a primary description of refraction will be considered. For 

 a more complete study of refraction of light, the writer would refer the 

 student to some good work on physics. 



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