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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the equation 



H'F' = ^ (F//) = FjV = - (M'FO 

 n n 



where n' is the index of the vitreous and n the index 

 of air. 



SURFACE 



FIG. 3. Snell's law of refraction. 



Refracting Power of the Eye 



Since the locations of principal points and the ratio 

 n'/n are relatively fixed from eye to eye and remain 

 relatively unchanged as a given eye accommodates, 

 the specification of the distance H'F' tells us prac- 

 ticalh' all we need to know about the refracting 

 mechanism of an eye. Instead of specifying this dis- 

 tance, which is known as the secondary focal length, 

 it is more usual to specify the refracting power F, 

 but 



F = n'/H'F'. 



The refracting power is expressed in diopters. 



The refracting power of an eye is dependent upon 

 the curvature of the refracting surfaces and the indices 

 of the media. In all cases except for the back surface 



FIG. 4. Cardinal points of the GuUstrand schematic eye. In this eye the secondary focus F' falls 

 0.387 mm behind the retina, i.e. the layer of photosensitive elements that respond to light. The 

 principal points //and //'are located 1.348 mm and 1.602 mm from the front surface of the cornea, 

 respectively. FH = M'F' = 17055 mm and H'F' = FJV = 22.785 mm. The ratio H'F'/FH is 

 equal to the index of the vitreous. 



FIG. 5. Ray tracing. 



