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should receive attention. Every person has un- 

 questionably observed that when a spoon is placed 

 in a tumbler of water it is apparently bent at the 

 surface of the water, or when looking at an object 

 lying in the bottom of a dish, it is apparently at a 

 different point than when viewed from the side. 

 This is caused by the great principle of actually 

 bending the rays of light as they pass from one 

 transparent medium into another of greater or less 

 density and is called refraction. The amount of 

 refraction increases as the difference in the density 

 of the two media becomes greater. We also know 

 that in viewing objects through a glass prism, they 

 apparently lie in a different direction than their 

 real one. The amount of this deviation is absolute 

 and depends for the one factor upon the density of 

 the glass composing the prism and for the other 

 upon its shape. 



Fig. i. 



Fig. 1 represents a cross section of a prism 

 and shows how the ray, as it touches the first 

 surface of the prism, undergoes refraction and on 



