OPTICS 265 



thus shortened. This slackens the zonule and the capsule 

 of the lens, and the lens, in consequence of its elasticity, 

 bulges forward so that its curvature becomes greater (Fig 

 30, N). 



The muscles of accommodation have smooth fibres. 



The oculo-motor nerve supplies them with motor fibres 

 which enter into the ciliary ganglion and thence proceed, as 

 the short ciliary nerves, to the eye. 



The accommodation muscles of both eyes are innervated 

 simultaneously and to the same extent. Simultaneously with 

 these the internal rectus and the sphincter iridis of both eyes 

 are innervated, so that accommodation is always accom- 

 panied by convergence of the eyes and contraction of the 

 pupils. 



Measure of accommodation. The point which the resting, 

 eye sees clearly is called the far point ; the point which the 

 most strongly accommodated eye sees clearly is called the 

 near point. The range of accommodation is the distance of 

 the far point from the near point. The power of accommo- 

 dation is measured by the reciprocal of the near point minus 

 that of the far point, hence for the normal eye, in which the 

 near point is o. 12 m, 



, or 8.3 diopters. 



0.12 so 0.12 



This number expresses the refractive power which a convex 

 lens must have in order to have the same dioptric effect as 

 the eye normally has during strongest accommodation. 

 The power of refraction (diopter) is expressed by the recip- 

 rocal of the focal length of the lens. A lens having the 

 focal length of I m has the value of I diopter. The power 

 of accommodation is therefore equal to that of a lens of o. 12 

 mm focal length. The power of accommodation decreases 

 with age, because the lens becomes hard (presbyopia). 



Anomalies of refraction (near-sightedness or myopia, far-sighted- 

 ness or hypermetropia) are due to abnormal positions of the retina. 

 In myopia the retina lies too far back, ami parallel rays meet in 



