412 ELEMENTARY PHYSIOLOGY less. 



that the tvx> images of the middle pair come nearer to- 

 gether when the eye is accommodated for a near object 

 than it is to observe the slight movement and diminution 

 in size of the single imarje of a candle flame. 



These may be regarded as the facts of adjustment with 

 which all explanations of that process must accord. They 

 at once exclude the hypothesis (1) that adjustment is the 

 result of the compression of the ball of the eye by its 

 muscles, which would cause a change in the form of the 

 cornea ; (2) that adjustment results from a shifting of the 

 lens bodily, for its hinder face does not move ; (3) that it 

 results from the pressure of the iris upon the front face of 

 the lens, for under these circumstances the hinder face of 

 the lens would not remain stationary. This last hypo- 

 thesis is further negatived by the fact that adjustment 

 takes place equally well when the iris is absent. 



But one other explanation remains, which is not only 

 exceedingly probable from the anatomical relations of the 

 parts, but is also supported by direct experimental evi- 

 dence. The lens, which is very elastic, is kept habitually 

 in a state of compression by the pressure exerted on it by 

 its suspensory ligament, and consequently has a flatter 

 form than it would take if left to itself. If the ciliary 

 muscle contracts, it must, as has been seen, relax that 

 ligament, and thereby diminish its pressure upon the lens. 

 The lens, consequently, will become more convex ; it will, 

 however, since it is highly elastic, return to its former 

 shape when the ciliary muscle ceases to contract, and 

 allows the choroid to return to its ordinary place. 



Hence probably the sense of effort we feel when we 

 adjust for near distances arises from the contraction of 

 the ciliary muscle. 



4. The Limits of Accommodation. Use of Spectacles. 

 — Adjustment can take place only within a certain 

 range ; this, however, admits of great individual varia- 

 tions. 



People possessing ordinary, or as it is called ' ' normal " 



