LABORATORY MANUAL OF PHYSIOLOGY. 



ADAPTATION OF THE EYE FOR DISTANCE. 



In the camera, however, it is necessary to adjust the instrument 

 by backward and forward movements of the lenses. In the eye, 

 this adjustment is brought about by changes in convexity of the 

 lens, or accommodation. Accommodation is, therefore, the func- 

 tional adaptation of the eye to distance. 



If the entire optical apparatus of the eye were rigid and fixed, 

 how could objects at various distances be seen clearly ? Explain 

 what takes place when the eye accommodates. 



Take 'a sharp-pointed pencil in each hand. With one eye 

 closed, hold the points in a direct line of vision before the other 

 eye one, about twenty centimetres distant, and the other a full 

 arm's length. Focus on the nearer pencil. Is the image of the 

 distant one clear ? Focus upon the farther pencil. Is the image 

 of the nearer one clear? 



The near point, or punctum proximum, is the nearest point to 

 the eye to which objects may be brought and still be seen clearly. 

 It averages about 1 2 cm. At this point the accommodation is most 

 active. 



Determine your own near point. 



The far point, or punctum remotum, is the farthest point at 

 which objects may be seen clearly by the normal eye. 



The range of accommodation is the difference between the punc- 

 tum proximum and the punctum remorum. 



Determine your own range of accommodation. 



ADAPTATION OF THE EYE FOR DIRECTION. 



As the eye can functionally adjust itself to distance, it can also 

 change the direction of its visual axis from one object to another, 

 or can follow objects moving within its field of vision. 



Two students may work together, one as observer and the other 

 as subject. 



(a) MONOCULAR FIXATION. The observer and subject being 

 seated opposite each other, let the subject close or screen one eye. 



