250 



HARDWICKE'S SCIENCE-GOSSIP. 



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 precisely identical. No 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 (Le9ons 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. 



great number of inverted partial images. How 

 then can insects and crustaceans see with their 

 compound eyes ? Midler 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 farther con- 

 centrates 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 the 

 brain combines these impressions into some kind of 

 picture, a picture like that which covdd be produced 

 by stippling. It may be added that the movements 

 of the insect's head or body would render the distance 

 and form of every object in view mucli readier of 

 appreciation. No accommodation for distance would 

 be necessary, and the absence of all means of accom- 



3. — Diagram 01 Insect integument, in sec- 



bm, basement-membrane ; hyp, hypo- Fig. 154. — Section through eye of Dytiscus-larva, showing the derivation of the 



lis, orchitinogenous layer ; ct, ci', chitinous parts Irom modified hypodermic cells. L, lens ; Cr, crystalline cones ; A', ncrve- 



le ; s,s. seta. rods ; N. Op, optic nerve. [From Grenacher.] 



Miillcrt in 1826 pointed out that so simple an ex- 

 planation was inadmissible. lie granted that the 

 simple eye, with its lens and concave retina, pro- 

 duces 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 jDigmented 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.* Lastly, 

 if this difficulty were removed, .Miiller thought it 

 impossible for the nervous centres to combine a 



* "Zur vergl. Phys. des Gesichtsinnes." 



■\ Exner has since determined by measurement and calculation 

 the optical properties of the eye of Hydrophilus. He finds that 

 the focus of a corneal lens is about 3mm. away, and altogether 

 behind the eye. 



niodation ceases to be perplexing. Such is Miiller's 

 theory of what he termed "mosaic vision." Many 

 important researches, some contradictory, some con- 

 firmatory of Miiller's doctrine,* have since been 

 placed on record, with the general result that some 

 modification of Miiller's theory tends to prevail. 

 The most important of the new facts and considera- 

 tions which demand attention are these : — 



Reasons have been given for supposing that images 

 are formed by the cornea and crystalline cones to- 

 gether. This was first pointed out by Gottsche 

 (1852), who used the compound eyes of flies for 

 demonstration. Grenacher has since ascertained 

 that the crystalline cones of flies are so fluid that 

 they can hardly he removed, and he believes that 

 Gottsche's images were formed by tlie corneal facets 



* A critical history of the whole discussion is to be found in 

 Grenacher's " Seh-organ der Arthropoden " (1S79), from which 

 we take many historical and structural details. 



