860 THE SENSE OF SIGHT 



divergent rays are focalized far behind the lens and may, therefore, fall 

 precisely upon the retina of the myopic eye. 



In order to enable a myopic person to see distant objects clearly, 

 we must lessen the convergence of the posterior bundle of the rays of 

 light, i.e., we must force their focal point farther backward until they 

 reach the retina. How can this be done? By rendering the enter- 

 ing rays more divergent, so that they impinge upon the lens more 

 widely separated from one another than formerly. The ordinary 

 efforts of the ciliary muscle will then suffice to centralize these more 

 divergent rays precisely upon the retina. Consequently, the condition 

 of myopia necessitates the use of concave lenses of a diverging power 

 exactly proportional to the degree of the myopia (Fig. 459). 



Hypermetropia. — The condition of hypermetropia or far-sightedness 

 is due either to a decrease in the longitudinal diameter of the eyeball 

 or to a diminution in the refracting power of the lens and other media 

 of the eye. ' The former is the most common cause. A hypermetropic 

 eye is unable to focalize rays emitted by near objects, because its 

 refractive mechanism is not sufficiently powerful to converge these 

 rays in a way to bring them to a sharp intersecting point upon the 

 retina. Since they are still too widely separated when they strike 

 this receptor, they cannot give a clear visual impression. In the more 

 severe cases, this statement also applies to the parallel rays, so that 

 even distant objects cannot be seen distinctly when the eye is at rest. 

 It is commonly said, that the focal point in the hypermetropic eye lies 

 behind the retina, but naturally, this is only a theoretical possibility. 

 With the increasing hypermetropia, the near point constantly moves 

 farther away from the eye, sometimes as far as 200 cm., while its far 

 point lies at an infinite distance. 



It will be seen, therefore, that the hypermetropic, as well as the 

 myopic eye, when at rest, sees distant objects indistinctly. Contrary to 

 the myopic eye, however, the hypermetropic organ is able to overcome 

 this difficulty for a time by constantly making extra efforts at accom- 

 modation. It is evident that any slight shortening of the eyeball 

 may be compensated for by rendering the lens unusually convex, but 

 naturally, these hyperefforts must fail to produce the desired result if 

 the shortening has progressed beyond the limit of accommodation. 

 Besides, these forceful contractions of the ciliary muscle are generally 

 followed by a strained feeling, orbital pain, headache, and vertigo. 

 Slight degrees of hypermetropia, however, may never be noticed for 

 the reason that the person so affected may readily overcome them 

 by a somewhat greater contraction of the ciliary muscle. In the 

 course of time, this muscle then frequently undergoes a compen- 

 satory hypertrophy. 



The condition of hypermetropia may be remedied by forcing the 

 focal point farther toward the lens; i.e., by rendering the rays of 

 light emerging from the posterior surface of the lens, more convergent. 

 How can this end be accomplished? By supplying the lens with 



