ORGANS OF THE SENSES. 33 



hear that the eye of the common House-fly presents 

 4000 of these facets ; but even this number sinks into 

 insignificance when compared with the 17,000 facets 

 observed in the eye of a species of Butterfly, or the 

 25,000 in that of a smaU Beetle. 



Nor is our wonder lessened when we come to learn 

 the remarkable perfection of structure exhibited by 

 each of these minute visual organs. Each of the 

 facets that we see on the surface of the eye of an 

 insect, is a small horny lens, which is usually flat 

 upon the surface, but exhibits a considerable con- 

 vexity at its inner extremity. The centre of this 

 convexity touches the centre of the base of a small 

 crystalline cone, and this in its turn is enveloped by 

 the extremity of a minute nervous thread, which 

 passes up to it from the optic lobes of the brain. 

 Nor is this all, for the delicate nerve is enveloped 

 throughout, and entirely separated from its fellows 

 by a sort of case, within which are numerous grains 

 of pigment extending up to the point where the outer 

 horny lens and the crystalline cone come in contact, 

 so as to prevent the light from passing through 

 anywhere except just at the centre of the eye. 

 When we consider that amongst all these delicate 

 parts, minute muscular threads are to be found, whose 

 office it is doubtless to produce some change to adapt 

 each to particular necessities of vision, we shall be 

 quite ready to admit that few things in this world 

 are more wonderfully constructed than the eyes of 

 Insects. 



To the organs just described the name of compound 

 eyes is given ; but as though these were not sufficient, 

 nature has furnished many insects with two or three 

 simple eyes (or ocelli] , which are generally situated 



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