266 SCIENCE OF COMMON THINGS. 



Use of the e} r ebrows and eyelashes. 



are held farther off, it compensates for this defect, and 

 a perfect image is formed on the retina. 



Birds of prey are enabled to adjust their eyes so as to see objects at a 

 great distance, and again those which are very near. The first is accom- 

 plished by means of a muscle in the eye, which enables them to flatten 

 the cornea by drawing back the crystalline lens ; and to enable them to 

 perceive distinctly very near objects, their eyes are furnished with a 

 flexible bony rim. by which the cornea is thrown forward at will, and the 

 eye thus rendered near-sighted. 



1*710 Why do persons who are short-sighted in youth, gradually have 

 this failing corrected as they grow old ? 



They are short-sighted because the cornea of the eye 

 is too globular ; but as age advances, the fluids are not 

 secreted as before, the eye becomes flattened, and 

 natural sight is again restored. 



1730 What is the use of the eyebrows ? 



The eyebrows defend the eyes from too strong a light, 

 and serve to turn away substances which might other- 

 wise fall into the eye. 



1731 What is the use of the eyelashes? 



The eyelashes guard the eye from danger, and pro- 

 tect it from dust or insects floating or flying in the atmo- 

 sphere. 



17*33 Why is the eye pained by a sudden light f 



Because the nerve of the eye is burdened with rays 

 before the pupil has had time to contract. 



1733 What is the pupil of the eye ? 



The circular black opening in front of the eye. 



1734 Why does it give us pain if a bright light is brought suddenly 

 towards us at night-time ? 



Because the pupil of the eye dilates very much in 

 the dark in order to admit more rays. 



When therefore a light is brought suddenly before us, the enlarged 

 pupils overload the optic nerves with rays, which causes pain. 



1735 Why can we bear the light Rafter a few moments f 



Because the pupils contract again almost instantly, 

 and adjust themselves to the quantity of light which 

 falls upon them. 



1736 Why can we see nothing when we leave a well-lighted room, and 

 go into the darker road or street ? 



