ITS NERVOUS SYSTEM AND SENSE ORGANS. 105 







in the field of a microscope, gives images of surrounding objects, 

 and that these images are inverted. When the cornea is 

 flattened out for microscopic examination, the images (e.g., of a 

 window or candle-flame) are similar, and it has been too hastily 

 assumed that a multitude of identical images are perceived by 

 the Insect. The cornea of the living animal is, however, 

 convex, and the images formed by different facets cannot be 



CJ / 



precisely identical. JN"o combined or collective image is formed 

 by the cornea. When the structure of the compound eye had 

 been very inadequately studied, as was the case even in Cuvier's 

 time (Lecons d'Anat. Comp., xii., 14), it was natural to suppose 

 that all the fibres internal to the cornea were sensory, that they 

 formed a kind of retina upon which the images produced by the 

 facets were received, and that these images were transmitted to 

 the brain, to be united, either by optical or mental combination, 

 into a single picture. Miiller,* in 1826, pointed out that so 

 simple an explanation was inadmissible. He granted that the 

 simple eye, with its lens and concave retina, produces a single 

 inverted image, which is able to affect the nerve-endings in the 

 same manner as in Vertebrates. But the compound eye is not 

 optically constructed so as to render possible the formation of 

 continuous images. The refractive and elongate crystalline 



*/ 



cones, with their pointed apices and densely pigmented sides, 

 must destroy any images formed by the lenses of the cornea. 

 Even if the dioptric arrangement permitted the formation of 

 images, there is no screen to receive them.f Lastly, if this 

 difficulty were removed, Miiller thought it impossible for the 

 nervous centres to combine a great number of inverted partial 

 images. How then can Insects and Crustaceans see with their 

 compound eyes? Miiller answered that each facet transmits a 

 small pencil of rays travelling in the direction of its axis, but 

 intercepts all others. The refractive lens collects the rays, and 

 the pigmented as well as refractive crystalline cone further 

 concentrates the pencil, while it stops out all rays which diverge 

 appreciably from the axis. Each element of the compound eye 

 transmits a single impression of greater or less brightness, and 



Exner has since determined by measurement and calculation the optical 

 properties of the eye of Hydro philus. He finds that the focus of a corneal lens is 

 about 3mm. away, and altogether behind the eye. 

 f Zur vergl. Phys. des Gesichtsinnes. 



