The Evolution of Optics. 269 



agent to focus them. To accomplish this purpose the eye 

 possesses the power of accommodation corresponding to the 

 focusing apparatus of our camera. This is effected by 

 means of the crystalline lens, which hangs suspended, as we 

 have seen, behind the iris, and upon which the suspensory 

 ligament exercises a constant traction when the eye is in a 

 state of rest. At the attachment of this suspensory liga- 

 ment is situated a muscle which surrounds the lens. When 

 we wish to adjust the eye for near objects this muscle con- 

 tracts, and, as a necessary result, the traction exercised upon 

 the suspensory ligament of the lens is relieved, and by its 

 own resiliency the lens assumes a more convex shape, just 

 as a rubber ball upon which you have been pulling if re- 

 leased springs back to its original form (see Fig. 2). The 

 convexity of the lens is thus increased, and consequently its 

 refracting power, so that the diverging rays of light are 

 now focused upon the retina, the accom- 

 modation effort being exactly propor- 

 tioned to the nearness of the object upon 

 which the eye is directed. 



This in its simplest form is the mech- 

 anism of accommodation. 



The eye which we have thus far con- 

 sidered is the normal or ideally perfect 

 eye. But, as met with in actual experi- 

 ence, eyes vary widely from this stand- 

 ard. The most common defect is that 

 known as hypermetropia, or far sight, 

 the latter term being somewhat mislead- 

 ing, because such eyes do not always 

 possess perfect vision at long range, nor 

 is their vision for near objects necessa- 

 rily imperfect. A far-sighted eye is 

 simply a flattened eye (Fig. 3), and the 

 retina is situated in front of that point 

 to which the rays of light are converged 

 when in a state of rest. This defect is most usually com- 

 pensated by a constant accommodation effort of the focusing 

 muscle and the individual is not conscious of any visual 

 defect. 



Myopia, or near sight (Fig. 3), is that condition in which 

 the length of the eye is too great and the rays of light are 

 focused in front of the retina. With such an eye it is im- 

 possible to obtain a clear picture of distant objects, and the 



