THE EYE. 245 



LESSON XVI. 



THE EYE VISION. 



WHEN we look at the eye, we see that the front 

 part of it is bright and transparent, and that behind 

 this there is a dark-looking curtain, with an open- 

 ing in its centre. The bright part is the cornea, 

 and is fixed into what we call the white of the 

 eye, very much in the same way that a watch-glass 

 is fixed into its case. The curtain is named the 

 iris, and the opening through it the pupil. 



The iris is a very delicate circular muscle, and 

 its colour is owing to a dark paint which covers it 

 behind, and which easily washes off. The action 

 of the iris is seen if we bring a candle close to the 

 eye; the pupil contracts closer and closer, accord- 

 ing to the brightness of the light, and enlarges 

 again as it is removed. In order that its action 

 may be perfectly free, the part in which the iris 

 moves is filled with a watery fluid, called the 

 aqueous humour. 



Farther back, in the ball of the eye, are other 

 curious parts, as the crystalline lens, which is in 

 shape just like a small glass in a telescope, and 

 is placed exactly behind the pupil ; and a third 

 humour called the vitreous humour. 



All these parts are made like an optical instru- 

 ment, to conduct and gather the rays of light. 

 They are subservient to another part called the 



