MECHANISM OF ACCOMMODATION. 569 



mal eye, called the emmetropic eye, this " near limit'' is about 12 

 cm., or 5 inches, but it varies in different individuals. 



For objects that are over 10 metres distant very little change 

 in the eye is required in order to see them distinctly, and the 

 nearer the object approaches the more frequently the adjustment 

 of the eye has to be altered in order to see it clearly. But at 

 every part of the range of distinct vision objects at different dis- 

 tances can be seen without moving the adjustment. The range 

 of this power is measured on the line of vision, and called the 

 focal depth. In the distance we can take in a greater depth of 

 landscape, and this without effort or fatigue ; but when looking 

 at near objects we must constantly accommodate our eyes afresh 

 in accordance with the shallowness of our focal depth. 



The mechanisms by means of which the accommodation of the 

 eye is accomplished, differ from anything that can be applied to 

 an artificial optical instrument, and are much more perfect. 



The changes which have been observed to take place are : (1) 

 The iris contracts so that the pupil becomes smaller; (2) the cen- 

 tre of the anterior surface of the crystalline lens moves slightly 

 forwards, pushing before it the pupillary margin of the iris, and 

 becomes more convex ; (3) the posterior surface of the lens also 

 becomes more convex but without changing its position. 



These changes can be seen in the accompanying diagram, show- 

 ing a section of the lens, cornea, and ciliary region (Fig. 225), 

 in the left-hand side of which the lens is drawn in the position 

 it assumes when accommodated for near objects. These move- 

 ments can be seen to take place in life by observing the changes 

 in relative positions, etc., of the reflections of a candle flame 

 thrown from the cornea and the two surfaces of the lens. On 

 the cornea is seen a bright upright flame ; next comes a large 

 diffused reflection from the anterior surface of the lens, and at the 

 other side of this a small inverted image of the flame reflected 

 from the posterior surface of the lens. When the adjustment is 

 changed by looking from a far to a near object, the image on the 

 front of the lens becomes smaller and moves towards the centre 

 of the pupil. The image on the back of the lens also becomes 

 smaller but does not change its position. The exact amount of move- 



48 



