REFRACTION IN THE EYE 469 



ACCOMMODATION 



Accommodation. Look at any distant object. 



The object will be seen clearly. The (practi- 

 cally) parallel rays proceeding from the object 

 are brought to a focus on the retina. 



Look at an object ten inches from the eye. 



The rays proceeding from this object are evi- 

 dently divergent, yet the object is seen clearly. 

 The divergent rays have also been focussed on 

 the retina. This power of voluntarily bringing 

 divergent rays to a focus on the retina is termed 

 accommodation. v 



Scheiner's Experiment. With a fine 'needle 

 pierce in a card two holes at a distance from each 

 other a little less than the diameter of the pupil 

 (average 4 mm.). Hold the card with the holes 

 horizontal and near the pupil. Look through the 

 holes at a pin or needle held vertical about 15 cm. 

 (6 inches) in front of the eye. 



The needle will fee seen clearly. J 



Move the index finger over one of the holes. 



There will be no change except that the visual 

 field will be darker. 



Fix a distant object, for example, a cloud. 



The needle will appear double, and each image 

 will be rendered indistinct by dispersion circles. 



Move the index finger over the left-hand hole. 



