THE CILIARY BODY AND LENS 829 



parallel rays coming from the distance, its far point must lie at the 

 horizon. Consequently, its location must vary with those outside 

 factors upon which the visibility of objects ordinarily depends. Practi- 

 cally, however, it has been found that an eye can also perceive objects 

 without accommodation which are situated at a distance 'of only 6 

 to 10 m. from it. The deduction to be drawn from this fact is that 

 even objects situated at this short distance, emit a large number of 

 parallel rays which the eye is able to intersect upon the retina without 

 actually increasing its refractive power. In this action, however, 

 the different refractive media are aided by the material depth of the 

 receptor. In other words, the rods and cones of the retina upon 

 which the light impinges, do not form a true plane but possess a certain 

 depth, allowing us to vary the anterior focal distance in a shght 

 measure without actually causing the posterior focal point to fall 

 entirely outside this layer. 



If the object is now moved nearer to the eye than the aforesaid 

 distance, the mechanism of accommodation is immediately brought 

 into play with the result that the now divergent rays are still brought 

 to a focus upon the retina.^ The closer the object is made to 

 approach the cornea, the greater will be this effort at accommodation 

 until its physiological limit has been reached. As has just been stated, 

 the nearest point at which the eye is still capable of forming a distinct 

 image, is called the near point. Beyond this inner limit the rays of 

 light emitted by an object, are so divergent that they can no longer 

 be brought to a sharp intersecting point upon the retina. The image 

 then appears merely as a diffused area of light which fails to give 

 a proper visual impression. 



The determinations of the near-point by means of the ophthal- 

 mometer have shown that it does not retain a constant position, but 

 varies not only with age, but also with the general condition of the 

 body and local defects in refraction. At birth, the lens is rather 

 spherical in shape; hence, the infant's eye should really be adjusted 

 for near objects, were it not for the fact that the eyeball is at this time 

 still too small. In reality, these two factprs are adjusted in such a 

 way that this eye is somewhat far visioned. Beginning at about the 

 age of 10 years, the near point recedes gradually with advancing years 

 ^nd more markedly between the fortieth and fiftieth year. This 

 observation is more fully illustrated by the following figures: 



Age 



10 years — 7 cm. in front of cornea, equalling 14 D. refr. power 



20 years — 10 cm. in front of cofnea, equalling 10 D. refr. power 



30 years — 14 cm. in front of cornea, equalling 7 D. refr. power 



40 years — 22 cm. in front of cornea, equalling 4.5 D. refr. power 



50 years — 40 cm. in front, of cornea, equalling 2.5 D. refr. power 



60 years — 100 cm. in front of cornea, equalling 1.0 D. refr. power 



^ The term retina is employed here as well as elsewhere, although it is to be 

 clearly understood that we are actually referring to the sensitive inner layer of the 

 retina, namely to the rods and cones. 



