Spatially Determined Reactions 



219 



pound eye where light can pass from one section to another, 

 and where the image is formed by the cooperation of vari- 

 ous refracting bodies (324). 



The simplest and vaguest conceivable visual image would 

 be that of a visual field whose different parts should differ 

 in brightness. 

 An eye capa- 

 ble of furnish- 

 ing indications 

 merely of the 

 direction from 

 which the great- 

 est illumination 

 comes might 

 produce this 

 kind of an im- 

 age, which 

 would of course 

 not allow the 

 perception of 

 objects, only 

 that of bright- 

 ness distribution. The compound eye found in crus- 

 taceans and insects would seem to be adapted chiefly 

 for the perception of light direction and of moving stimuli. 

 It consists essentially of a number of simple eyes so crowded 

 together as to produce a common faceted cornea, each 

 facet belonging to an eye. These facets are lens shaped, 

 and back of each lies a refractile crystalline cone. Behind 

 these, in turn, are nervous structures, the rods or retinulas, 

 each separated from its neighbors by a pigment sheath. 

 Light rays passing through each corneal facet probably 

 produce a single spot of light on the retinula, and the 



FIG. ii. Diagrammatic representation of the compound 

 eye of a dragon-fly. C, cornea; K, crystalline cone; 

 P, pigment; R, nerve rods of retina; Fb, layer of 

 fibres; G, layer of ganglion cells; Rf, retinal fibres; 

 Fk, crossing of fibres. After Claus. 



