OF SENSATION AND THE SENSES. 487 



juices of flowers or other vegetable substances, they may probably be 

 of especial service to them for the discovery of this pabulum, parti- 

 cularly to those which thrust themselves into the flowers themselves, 

 and there seek the nectaries. 



The compound eyes of insects appear constructed for vision at greater 

 distances, and to embrace a wider horizon, and yet by means of these 

 only are they enabled to have a distinct close sight. They are so 

 composed that each individual facet can survey but a small space of 

 the entire field of vision, so that each contributes to the perception of 

 all the objects comprised within that field ; but each separate one 

 does not at the same time see all such objects, whence the insect must 

 receive as many forms of objects in its eye as there are individual 

 facets to the eye. This consequence of a common and yet subsidiary 

 vision of these facets springs partly from the immobility of the eyes, 

 and partly it arises from the circumstance that only those rays of light 

 which fall in a right line upon a facet of the eye, which itself forms 

 the segment of a circle, can reach the optic nerve of this facet, 

 whereas all others are withheld by the pigment which partly sepa- 

 rates the individual glass lenses * from each other, and partly circu- 

 larly surrounds the margin of the crystalline lens beneath the cornea. 

 Hence it results that the nearer the object is, the more obliquely do all 

 but the perpendicular rays of light fall upon the facet, and therefore 

 contribute so much the less to the production of the image ; the object 



* Treviranus, even in his latest work (Gesetze und Erscheinungen des Organischen 

 Lebens, neu dargestellt, vol. ii. Pt. 1, p. 77), denies the presence of this glass lens in the 

 eyes of all insects. Joh. Miiller has so far modified his earlier assertion that the Diptera 

 in lieu of it exhibit beneath the cornea a transparent crystalline layer, which beneath each 

 facet of the cornea stands in connexion with a filament of the optic nerve, but that in all 

 insects there is either a true crystalline lens or something analogous. According to 

 Treviranus, the lenses serve " to shorten the distance of the concentrated rays from the 

 divisions of the cornea to the extreme ends of the filaments of the optic nerve, there where 

 the light, owing to these divisions, is but slightly refracted, and the refracted rays form a very 

 long arch." But, according to Job. Miiller, their object appears to be rather to concentrate 



into one point the rays falling in a right line But the observation communicated by 



Treviranus is more important, namely, that the filaments of the optic nerve proceed at first 

 from the clavate optic nerve itself in large sterns, whence subsequently radiating branches 

 divaricate, as has been figured by Straus in the cockchafer (PI. IX. f. 6). In (Eschna 

 forcipata he even saw the nerves run parallely to the plate which forms the inner circum- 

 ference of the eye, and thence" proceeded the filaments destined to supply the divisions of 

 the cornea. 



