852 The Outline of Science 



It is a roundish ball made of dense and strong fibrous tissue, 

 opaque for five-sixths of its surface, but transparent in the one- 

 sixth which bulges out in front, as the cornea. To the interior of 

 the cornea, separated from it by a watery fluid, there is a delicate 

 curtain which hangs over the transparent "window" in front 

 and forms the variously coloured iris. 



This curtain is a wonderful arrangement for adapting the 

 eye to the intensity of light which falls on it. Fibres of muscles 

 are so ingeniously distributed in it that it can almost close the 

 opening in a strong light, or open it wide when the light is fainter. 

 The "iris diaphragm" with which the photographer regulates the 

 entrance of light into his camera is merely a poor imitation of it. 

 Moreover, it contains pigment cells, which may be crowded when 

 the light is strong or fewer in number when the eye wants as 

 much light as possible. So we get the black eyes (eyes rich in 

 pigment, to mitigate the light) of the southerner, the blue eyes 

 (with little pigment) of the dweller in the darker northern lands, 

 and every intermediate shade and combination of them. 



Behind the circular window, the pupil, is the crystalline lens, 

 which, unlike any lens that man can make, can be altered by fine 

 muscles so as to focus itself for any distance. Other muscles and 

 tendons are attached to the outside of the eyeball, and they 

 automatically turn it in the direction we want. Some men of 

 science have found many defects in the eye, and there are defects : 

 but when one thinks of the unconscious agencies that have built 

 up this wonderful camera, and work it automatically every 

 moment of our waking lives, one is not disposed to cavil. 



But the most wonderful part of it is the "sensitive plate" 

 at the back of the eyeball. A semi-transparent membrane, which 

 we call the retina, lines three-fourths of the interior of the eyeball 

 (which is filled with fluid), and it is particularly developed at 

 one spot, the real seat of distinct vision. On this "yellow spot" in 

 each eye the rays of light form an inverted image of the object 

 at which we are looking. The stereoscope enables us to under- 



