162 HUMAN PHYSIOLOGY. 



seen upon the retina. The size of the retinal image depends upon the 

 visual angle, which in turn depends upon the size of the object and its 

 distance from the eye. At a distance of 15.2596 metres the image of an 

 object I metre high would be I millimetre, or a thousand times smaller than 

 the object. 



Accommodation. By accommodation is understood the power which 

 the eye possesses of adjusting itself to vision at different distances. In a 

 normal or emmetropic eye parallel rays of light are brought to a focus on 

 the retina ; but divergent rays, that is rays coming from a near luminous 

 point, will be brought to a focus behind the retina, provided the refractive 

 media remain the same ; as a result vision would be indistinct, from the 

 formation of diffusion circles. It is impossible to see distinctly, therefore, 

 a near and distant object at the same time. We must alternately direct 

 the vision from one to the other. A normal eye does not require adjust- 

 ing for parallel rays ; but for divergent rays a change in the eye is necessi- 

 tated ; this is termed accommodation. In the accommodation for near 

 vision the lens becomes more convex, particularly on its anterior surface ; 

 the increase in convexity increases its refractive power ; the greater the 

 degree of divergence of the rays previous to entering the eye, the greater 

 the increase of convexity of the lens and convergence of the rays after 

 passing through it. By this alteration in the shape of the lens we are 

 enable to focus light rays coming from, and to see distinctly, near as well 

 as distant objects. 



Function of the Ciliary Muscle. Though it is admitted that the 

 change in the convexity of the lens is caused by the contraction of the 

 ciliary muscle and the relaxation of the suspensory ligament, the exact manner 

 in which it does so is not understood. When the eye is in repose as in 

 distant vision, the suspensory ligament is tense and the lens possesses that 

 degree of curvature necessary for focusing parallel rays. In the voluntary 

 efforts to accommodate the eye for near vision, the ciliary muscle contracts, 

 the suspensory ligament relaxes and the lens, inherently elastic, bulges for- 

 ward and once again focuses the rays upon the retina. It is, therefore, 

 termed the muscle of accommodation, and by its alternate contraction and 

 relaxation the lens is rendered more or less convex, according to the 

 requirements for near and distant vision. 



Range of accommodation. Parallel rays coming from a luminous 

 point, distant not less than 200 feet, do not require adjustment : from this 

 point up to infinity no accommodation is required for perfect vision. This 

 is termed \\iepunctum remotum, and indicates the distance to which an object 



