and smooth or simple Eyes of Insects, 279 



U, or at the expansion of this nerve, so that these filaments may 

 afterwards transmit to the brain the impression of the rays of 

 light, or the image. 



The great number of facets which compose the cornea, is no 

 obstacle to what we have said ; but each of these facets ought 

 to be considered as a cornea, which exercises its action on the 

 rays of light, and makes them undergo the changes necessary for 

 the brain to receive their impression. Nor ought their number 

 to make us suppose that insects ought to see c' jects very much 

 multiplied; for, whatever maybe the number of the nervous fila- 

 ments which correspond to each facet, they all set out from the 

 spreading of the optic nerve, which we have considered as a re- 

 tifia sufficiently analogous to that of the red-biooded animals : 

 it is on this retina that are painted the images perceived by the 

 filaments ; perhaps even this retina is only destined to transmit 

 to the brain the impression produced by the rays of light on the 

 optic nerves. This is the more probable, l>ecause images can 

 hardly be supposed to exhibit themselves from behind an opaque 

 membrane. The great use of the retina would therefore be to 

 centralize the impression, and to render it unique, if we may so 

 express ourselves. 



From what we have seen, if the force and number of the rays 

 of light had been very considerable, these rays might by their 

 too great excitabilitv injure the organ of sight. But nature, 

 equally admirable in her minutest details as in her most beautiful 

 works, has guarded against this inconvenience ; and the varnish 

 of the choroid, as well as the choroid itseif, are the organs 

 which hinder the multiplicity of the luminous rays from deran- 

 ging or altering the sensibility of the nervous filaments. The 

 varnish of the choroid and the memijrane of the choroid appear, 

 therefore, destined to absorb the excess of the luminous rays, and 

 to diminish the too great excitability wiiich a very strong light 

 would necessarily have produced on the optic nerves, accord- 

 ing to the conformation of the eves of insects. The black and 

 opaque bands which we observe in certain species, like the more 

 or less coloured spots in others, seem chiefly intended to absorb 

 the excess of t'ne luminous rays, while those less thick and more 

 transparent facilitate, on the contrary, the passage of the light. 



It is easy to j)erceive how necessary this arrangement was to 

 insects: in fact, the latter not having, like most of the vertebral 

 animals, pupils with the property of contracting and dilating, 

 and whicli, as a consequence of this arrangement, permit these 

 animals to receive only the ravs the most approximating the 

 perpendicular, or the axis of vision, had occasion for an opaque 

 membrane to absorb the useless ravs^ or such as might even be 

 S 4 ■ hurtful 



