THE SENSE OF VISION. 



755 



cated by the dotted line. The change in the appearance of the images is 

 represented diagram matically in Figure 220. On the left is shown the appear- 

 ance of the images as seen when the eye is at rest, a representing the corneal 

 image, 6 that reflected from the anterior, and c that from the posterior surface 

 of the lens when the observing eye and the candle are in the position repre- 



FIG. 220. Reflected images of a candle-flame as seen in the pupil of an eye at rest and accommodated 



for near objects (Williams). 



sented in Figure 219. The images are represented as they appear in the dark 

 background of the pupil, though of course the corneal image may, in certain 

 positions of the light, appear outside of the pupillary region. When the eye 

 is accommodated for near objects the images appear as shown in the circle on 

 the right, the image 6 becoming smaller and brighter and moving toward the 

 oorneal image, while the pupil contracts as indicated by the circle drawn round 

 the images. 



The changes produced in the eye by an effort of accommodation are indi- 

 cated in Figure 221, the left-hand side of the diagram showing the condition 



FIG. 221. Showing changes in the eye produced by the act of accommodation (Helmholtz). 



of the eye at rest, and the right-hand side that in extreme accommodation for 

 near objects. 



It will be observed that the iris is pushed forward by the bulging lens and 

 that its free border approaches the median line. In other words, the pupil is 

 contracted in accommodation for near objects. The following explanation of 

 the mechanism by which this change in the shape of the lens is effected has 

 been proposed by Helmholtz, and is still generally accepted. The structure 

 of the lens is such that by its own elasticity it tends constantly to assume a 

 more convex form than the pressure of the capsule and the tension of the sus- 

 pensory ligaments (s, s, Fig. 221) allow. This pressure and tension are dimin- 

 ished when the eye is accommodated for near vision by the contraction of the 

 ciliary muscles (c,-c, Fig. 221), most of whose fibres, having their origin at the 



