\ L\ THE OKGAXIC WORLD. 4^9 



the vertebrate plan. In some Butterflies and Dragon flies, each 

 &quot;compound eye&quot; is made up of many thousands of these &quot;eye 

 lets,&quot; the individual &quot; corneules &quot; of which give the &quot;facetted&quot; 

 appearance presented by the exterior of the aggregate mass ; whilst 

 the inner extremities of the cylinders abut upon a bulbous expan 

 sion of the optic nerve, from which a filament proceeds to each 

 of them. Now we seem fully justified by observation of the 

 movements of Insects, in concluding that these are guided by 

 visual perceptions of external objects not less distinct than our 

 own. And it seems probable, therefore, that the action of the 

 compound eye is to impress the sensorium of the Insect with a 

 single picture, corresponding to that which is formed upon our 

 own retina, though received through a very differently conducted 

 instrument Modern investigations, moreover, have shown that 

 the difference is rather apparent than real. For it is now known 

 that the retinal layer of the human eye is not a mere spreading- 

 out of the fibres of the optic nerve ; but that in front of these 

 terminal fibres is a layer of &quot; rods &quot; and &quot; cones &quot; on which the 

 retinal picture is formed. Thus, the visual picture which our 

 mind receives from either retina, is made up (so to speak) of the 

 aggregate of the visual impressions made separately and individ 

 ually upon each of its &quot; rods&quot; and &quot;cones,&quot; and through these 

 upon the individual fibres of the optic nerve on which they severally 

 impinge. And thus what may be called the &quot; mechanism &quot; of 

 our own vision, is really* analogous to that of the vision of the 

 Insect. In fact, it would now seem probable that the &quot;rods&quot; 

 and &quot;cones&quot; of our own retina are really homologous with 

 similar structures contained in the cylindrical &quot;eyelets&quot; of the 

 Insect ; so that the difference between its &quot;compound eye&quot; and 

 our own &quot;single eye&quot; lies only in the arrangement of the parts 

 of the recipient nerve-structure. Whilst we have a single relrac- 

 tive apparatus for the whole retinal area, by which a continuous 

 picture is thrown upon its entire expanse, the Insect has a sepa 

 rate refractive apparatus for each of its retinal elements ; but as 

 the retinal elements themselves are essentially the same in both 

 cases, we may fairly presume that the resulting visual sensation, 

 which the Insect receives by the combination of their separate 

 actions, corresponds closely with our own. That in the Insect 



