NATURAL HISTORY. 47 



And as lie spoke a big round drop 



F"l' bounding 1 on his ample sleeve ; 

 A witness mat he could not stop, 



A witness that we all believe." BLOOMFIELD. 



the light which penetrates the eye, make bodies in ignition appear during the 

 night as if they were surrounded with luminous rays. This appearance does not 

 take place if the eyelashes are inverted, or merely turned in another direction. 

 The vision of those persons who have lost their eyelashes is always more or less 

 imperfect. 



141. Why are the eyes protected by eyebrows ? 



The eyebrows have many uses. The projection which they form 

 protects the eye against external violence ; the hairs on account of 

 their oblique direction, and the oily matter with which they are 

 covered, prevent the perspiration from flowing towards or irritating 

 the surface of that organ : they direct it towards the temple and the 

 root of the nose. The colour and the number of hairs of the eye- 

 brows have an influence upon their use. They have generally some 

 relation to the climate. The inhabitants of hot countries have them 

 very thick and black ; the inhabitants of cold countries may have 

 them thick, but they are rarely black. The eyebrows protect tht 

 eyes from excess of light, particularly when it comes from above , 

 this effect is rendered still more conspicuous by the knitting of the 

 brows. 



142. Qf what use are eyelids? 



They cover the eye during sleep, and preserve it from the 

 contact of extraneous particles flying about in the air, which might 

 injure it ; they defend it from sudden shocks, by their almost 

 instantaneous closure, and by their habitual motions, which aro 

 renewed at nearly equal intervals, they preserve it from the effects 

 of long-continued contact of the air. The eyelids also moderate 

 the force of a too brilliant light, and prevent the passage of any 

 more of this fluid than what is necessary for vision without offend- 

 ing the eye. On the contrary, when the light is feeble, we separate 

 the eyelids to a, considerable distance, in order to admit the passage 

 cf as great a quantity of light as possible to the interior of the eye. 



143. Wh,y is the involuntary closing of the eyelid more 

 advantageous thin if it depended upon our will ? 



Because, if the clo'iin^ of the eyelid depended upon voluntary 



