Eyes 



41 



retinal cells. There can be little doubt, however, 

 that these simple eyes can only see objects a few- 

 inches away, and that the sight is much less clear than 

 that of the eyes of back-boned animals. With regard 

 to the compound eyes, it is supposed that each set 

 of facet, cone, and rod lets an exceedingly fine pencil 

 of light pass through, the surrounding dark pigment 

 absorbing all the oblique rays. Consequently each 

 element of a compound eye perceives only a very 

 small part of the field of vision, and the insect's view 

 of the outside world is made up of a vast number of 



Fig. 31. — Heads of Male (B) and Female (A) Honey-Bees, showing compound 

 and single eyes. Magnified 10 times. From Benton (after Cheshire), 

 Bull. I (n.s.) Div. Ent. U.S. Dept. Agr. 



small visual pictures. The supposed similarity of 

 such a view to the effect on our own sight of mosaic 

 work has led to the application of the term " mosaic " 

 to the vision of the compound eyes of insects. It is 

 evident that the extent of an insect's field of view 

 must depend on the number of facets in its compound 

 eyes and the approximation of the eyes to a globular 

 form, while the distinctness of the sight increases as 

 the size of the individual facets grows less. But the 

 widest difference of opinion prevails as to the perfec- 



