REFRACTION IN THE EYE 493 



will appear to be upright and of its natural size 

 (1.5 ram.). 



The apparent size of this image depends upon 

 a visual judgment The observer knows that 

 small objects are usually held about 250 mm. in 

 front of the nodal point. The size of an object 

 which at this distance would give a retinal image 

 1.5 mm. in diameter, can be found by formula 5, 



page 465. 



IS : 1.5 :: 250 : 15 



B 25 mm., the apparent size of the optic 

 disk viewed by the direct method. 



Ametropia ; Qualitative Determination. 1. 

 Let an assistant make the artificial eye ametropic 

 by moving the draw-tube until the optical axis is 

 shorter or longer than normal. The observer 

 should not know which form of ametropia has 

 been produced. Examine the retina with the 

 ophthalmoscope held from 30 to' 50 cm. in front 

 of the artificial eye. 



If the details of the ftmdus can be seen, the 

 eye is either myopic or hypermetropic. 



Move the head with the ophthalmoscope from 

 side to side. 



If the vessels appear to move in the same 

 direction, the eye is hypermetropic; if in the 

 opposite direction, the eye is myopic. 



Measurement of Myopia. The accommoda- 



