CHAPTER XX 



OUTPOSTS OF THE INTELLIGENCE SERVICE 



(c) DISTANCE RECEPTOR FOR LIGHT 

 THE EYE 



By W. F. SHANKS, B.Sc., M.B., Lecturer on Experimental Physiology in the 

 University of Glasgow. 



THE following points in connection with light are recalled to 

 the student's memory. 



(1) Refraction. 



When a ray of light passes from a rare to a dense medium 

 (or vice versa) it undergoes refraction, i.e. it is bent towards 



FIG. 46. Refraction of incident ray PO at interface AB. 



(or away from) the perpendicular to the surface at the point of 

 incidence. This perpendicular is called the normal. Snell's Law 



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