Visual Organs of Insects and Crustacea, 367 



The convexity of each of the different facets of the cornea 

 will refract and cause to converge towards the axis of each 

 corresponding cone the rays of light which come in the direc- 

 tion of this axis, and will so make them approximate more 

 closely towards each other in the interior of the eye. The 

 rays which illuminate the whole cone will thus be concentrated 

 to a single point or focus at the apex, and in this concentrated 

 state will impinge upon the optic filament; a circumstance 

 which will necessarily induce greater vividness of the image 

 or impression. But this refracting power of the external and 

 convex surface of the cornea is not so great as to cause each 

 individual facet to form a minute and separate image. Than 

 this, nothing, indeed, could be more adverse to the accuracy 

 of the visual sensation : for, if images were formed at the 

 focus of each facet, the facets acting as so many refracting 

 media or lenses, all the distinct images so produced would 

 necessarily be reversed, without a corresponding reversal of 

 the visual field taken as a whole ; the relative position, with 

 respect to each other, of the images of all the facets would be 

 exactly contrary to the relative position of the corresponding 

 points in the external object. Very frequently the facets of 

 the compound eyes have scarcely any convexity ; and, conse- 

 quently, we ought not to attribute to their external and con- 

 vex surface, even when the convexity is at its maximum, any 

 other effect than that of approximating and concentrating 

 towards the apex of the cones those rays which, according 

 to the known laws of the distribution of light, diverge in their 

 approach towards each of the cones. 



At present no satisfactory reason has been given to explain 

 the use of the different layers of pigment between the cones 

 and optic filaments. The pigment between the filaments 

 generally disappears gradually towards the bulb of the optic 

 nerve ; its particular use is to insulate the filaments from each 

 other. The filaments themselves are not always arranged in 

 straight lines ; but their course, from the apices of the cones 

 to the bulb of the nerve, is often slightly curved : and, without 

 the interposition of the pigment, the rays passing along the 

 axis of any one cone might, and probably would, affect simul- 

 taneously several of the filaments which are near to each 

 other, for they are not perfectly opaque. 



The compound eyes seeing only those objects which pre- 

 sent themselves in the axes of the cones, it must result that 

 the limits of their field of vision will be formed by the pro- 

 longation of the lateral boundaries of the eye itself. Thus the 

 visual field is greater or smaller, not in proportion to the 

 absolute magnitude of the eye, but as the form of it is more 



