90 THE BEAUTIES OF NATURE chap. 



"compound" eyes one on each side of the 

 head, have between them three small ones, 

 known as the " ocelli," arranged in a triangle. 

 The structure of these two sets of eyes is 

 quite different. The ocelli appear to see as our 

 eyes do. The lens throws an inverted image 

 on the back of the eye, so that with these 

 eyes they must see everything reversed, as we 

 ourselves really do, though long practice 

 enables us to correct the impression. On the 

 other hand, the compound eyes consist of a 

 number of facets, in some species as many as 

 20,000 in each eye, and the prevailing 

 impression among entomologists now is that 

 each facet receives the impression of one 

 pencil of rays, that in fact the image 

 formed in a compound eye is a sort of 

 mosaic. In that case, vision by means of 

 these eyes must be direct ; and it is indeed 

 difficult to understand how an insect can 

 obtain a correct impression when it looks at 

 the world with five eyes, three of which see 

 everything reversed, while the other two see 

 things the right way up ! 



On the other hand, some regard each 



