ACCOMMODATION. 167 



Helmholtz maintains that the anterior surface of the crys- 

 talline increases in convexity in looking at objects near at 

 hand, and flattens when looking at a distance; the pupil 

 contributes also to the accommodation by contracting in 

 looking at objects near at hand, and dilating to see at a 

 great distance. Nothing positive is known regarding the 

 manner in which this change of form in the crystalline is 

 effected. M. Helmholtz inclines to the opinion that the 

 diameter of the lens is increased or diminished, and conse- 

 quently it becomes more convex, or more flattened, accord- 

 ing as the zone of Zinn, which is inserted into the crystalline 

 capsule, is distended or relaxed by the action of the ciliary 

 muscle. 



Some simple experiments prove that the eye cannot see 

 distinctly, without an effort of adaptation, two objects placed 

 at unequal distances, and that the image distinctly perceived 

 by the retina when placed in the focus, is so no longer when 

 the focal distance is changed. 



i. If we look with one eye at the heads of two black pins, 

 placed in a line at the same level, but at different distances, we 

 shall see one of them distinctly and the other vaguely. If we 



Fig. 39. Accommodation of the eye to different distances. 



look at the nearest one the image is perfectly clear, while the 

 one farthest away is enveloped in mist; but if we look at the 

 latter we see it easily without change of position, but when its 

 image is well denned that of the other pin becomes confused. 



2. In looking at a pin through a small hole pierced in a 

 card, we can see either the pin or the edge of the hole dis- 

 tinctly; but when the image of one is distinct the other is 

 confused. 



3. By making two pin-holes through a card, at a distanc \ 



