REFRACTION IN THE EYE 465 



visual angle draw a second arrow parallel to the 

 first and twice its distance from the nodal point. 

 Produce the visual lines from the nodal point to 

 the retina. 



Observe that the retinal images of the large and 

 the small arrow are of equal size. The two ob- 

 jects subtend the same visual angle. Thus the 

 apparent size of an object depends upon the visual 

 angle. 



Size of Retinal Image. In the emmetropic eye 

 ( the eye accommodated for distant vision ) the 

 size of the object B is to the size of the retinal 

 image b as the distance from the object to the 

 nodal point of the reduced eye, gi, is to the dis- 

 tance from the nodal point to the retina, g^ 



(5) 



The retinal image is smaller than the object by 

 the number of times g 2 15 mm. is contained in 

 the distance, in millimetres, of the object from the 

 nodal point. 



Calculate the size of the retinal image of a 

 post one metre high placed 300 metres from the 

 observer's eye. 



Acuteness of Vision. Draw upon the visual 

 axis of the reduced eye a series of arrows of equal 

 size, each bisected by the axis. Draw lines from 



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