On Light in some of its Relations to Disease. 161 



In the figure at A we see rays from a distance, 



parallel rays, refracted in such a manner as to be brought 



to a focus at the retina. This is the normal eye. At B 



the ravs at the same distance from the cornea have not 

 t/ 



reached the retina, but cross over and are separated before 

 reaching it, so that a clear image of distant objects is not 

 formed. This is the near-sighted eye, and is technically 

 called a myopic eye. It is too long. At C we have the 

 reverse of this condition. Here the eye is too short, and 

 the diagram shows that the rays when they reach the 

 retina are not yet brought to a focus, but could they pass 

 through would unite in a focus behind the eye. This is 

 the eye commonly called oversighted — technically, hy- 

 permetropic. 



The oversighted person, in order to see even distant 

 objects clearly, must begin to exert the faculty of ac- 

 commodation. By contracting the ciliary muscle, such a 

 person is enabled to see a distant object with either per- 

 fect or comparative distinctness. The force, if the eye is 

 not much shortened, may not be more than the person 

 with the normal eye exerts when he looks at a near point; 

 but we see that, as the hypermetropic person brings the 

 object near his eye, he must exert much more than the 

 natural force in order to bring the rays to a proper focus. 

 As this muscle is forced to contract by nervous influence, 

 it will be seen that here is a source of nervous exhaus- 

 tion, especially if the muscle is forced to a continued con- 

 traction, as is the case when such a person reads or works 

 at needle-work for a considerable time. 



Thus it will be seen that, while normal eyes are always 

 at rest when accommodated for distance, and only slightly 

 exerted when accommodated for the near point, hyper- 

 metropic eyes are never at rest except when closed ; and 

 that, in viewing near objects, an excessive efibrt is required, 

 and that this excessive effort is increased in proportion as 

 the hypermetropia is more considerable. 



Trans, w:.] 21 



