THE SENSES 



543 



Accommodation. — All rays of light proceeding from a distant 

 object may be regarded as parallel, and all those proceeding 

 from an object within 6 metres (20 feet) of the eye may be 

 regarded as divergent. A distant object is one situated any- 

 where between 20 feet from the eye and infinity ; an object closer 

 than 20 feet to the eye is called near, and this point increases 

 up to 10 to 13 centimetres (4 or 5 inches), at which distance no 

 object c*an any longer be distinctly seen. The nearest distance 

 at which objects can be distinctly seen is called the near point. 

 Parallel rays need no focussing on the retina other than that 

 provided by the cornea ; but rays from near objects require 

 focussing owing to their divergent nature, and it is evident that 

 the nearer the object to the eye the greater the focussing required. 



FAR NEAR 



Fig. 162. — Diagram to illustrate Accommodation (Foster, after 

 Helmholtz). 



C.P., Ciliary process ; I, iris ; Sp. 1., suspensory ligament ; l.c.ra., longitudinal 

 ciliary muscle ; c.c.m., circular ciliary muscle ; c.S., canal of Schlemm. The 

 left half represents the shape of the lens for viewing distant objects, and the 

 right half that for viewing near objects. 



This focussing is brought about by a change in the shape of the 

 anterior surface of the lens ; it becomes more convex for near 

 objects, and this increase in convexity is due to the ciliary 

 muscle drawing forward the choroid coat, and with it the ciliary 

 processes. By this means the tension normally exercised 

 through the zonule of Zinn (the suspensory ligament of the lens) 

 is relaxed, and the lens of its own inherent elasticity bulges 

 forward, and so increases the curvature of its anterior face 

 (Fig. 162). A more convex lens is a more convergent one, and 

 its focus is therefore shorter ; in this way the images of near 

 objects are brought to a focus on the retina and distinctly seen, 

 whereas if this increase in curvature had not taken place, the 

 image would have been focussed behind the retina. The power 

 the eye possesses of focussing itself is known as the mechanism 



