THE EYE 



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upon the retina, yet by passing it through certain sub- 

 stances the rays become visible to the eye. Bone is pene- 

 trated by the rays with greater difficulty than flesh, and 

 so they can be photographed and seen within the body. 

 These rays cannot be bent from their course, and so can- 

 not be brought to a focus to produce a real image; 

 but images of objects are formed in shadows, due to the 

 varying degrees of light which passes through different 

 objects. 



The rays are sometimes called Rontgen rays from their 

 discoverer, and sometimes simply X rays because of their 

 unknown nature. Practical use of the rays is made in 

 looking within the body so as to determine by sight the 

 condition of the bones and the location of substances im- 

 bedded in the flesh. 



A foot in a shoe. 

 (From an X ray photograph.) 



