CHAP. XIX.] ORGANS OF SPECIAL SENSE. 239 



Perception of the figure of external objects therefore depends 

 on the action of the crystalline lens in converging all the rays, 

 emanating from a given point, to a focus on the retina. When 

 the lens of the eye is too convex, and its refractive power 

 excessive, the rays of light converge too soon and cross one 

 another before reaching the retina , consequently, the image 

 produced is not concentrated and distinct, but, dispersed moi e 

 or less over the surface of the retina, is diffused and dim. On 

 the other hand, if the lens 

 is too flat, the rays do not 

 converge soon enough, and 

 the image is again diffused 

 and indistinct. To remedy 

 a too great convexity of the 

 lens in the short-sighted or 

 eye, concave specta- 

 are used to disperse the 

 rays ; to remedy the flattened 

 lens in the Irypermetropic^or 

 long-sighted eye, we employ 

 convex glasses to concen- 

 trate and focus the rays 

 more quickly. 



A normal eye is capable 



Of distinct vision through- FlG< i 38 ._ DIAGRAM ILLUSTRATING RAYS 



out an immense range. We OF LlGHT CONVERGING IN A NORMAL EYE, 



.. (A), A MYOPIC EYE, (B), AND A HYPER- 



can see the stars millions of METROPIC EYE (C). 

 miles away, and with the 



same eye, though not at the same time, we can see objects within 

 a few inches of us. To be able to see objects millions of 

 miles away and within a short range, the eye has to accom- 

 modate or adjust itself to different distances. This ac- 

 commodation is accomplished mainly by the lens changing its 

 convexity. In accommodation for near objects, the lens becomes 

 more convex and the pupil of the eye likewise contracts. This 

 convexity is brought about by muscular effort, 1 and is always 

 more or less fatiguing. The accommodation for distant objects 

 is a passive condition, the convexity of the lens being unaltered 



1 Connected with the lens are tiny muscles, the ciliary muscles, contraction 

 of which alters the shape of the lens. 



