and smooth or simple Eyes of Insects. 285 



exists between the projection of thi^ suiooth eyes and tlieir small- 

 ness. 



The light arriving therefore -on the external membrane of the 

 smooth evcsj or ujion their coraea, undergoes various changes 

 which are connected with the fmm of the cornea. If it i)e con- 

 vex, as most generally observed, and the direction of the rays 

 be oblique, by making a very wide anj^le, they are reflected, and 

 do not traverse it. If the light, en the contrary, falls on the 

 cornea at a convenient angle, its rays ]}as3 through that mem- 

 brane, and undergo from it u retraction \''hic!i brings them near 

 to the perpeii licular. Finally, when the corr.ea is plain, the 

 rays of lighi will co:itiiuie their roivte, by approaching the per- 

 pendicuiar at the point ofiminersion ; whereas, if it were concave, 

 supjjosjing even tliat the luminous rays were parallel, they would 

 become divergent, and would remove fro;a the u.iis of vision. 



The light which has pavsed through tlie cornea, arrives im- 

 mediately on the ki'id of retina formed .by tiie optic nerve, a 

 continuation of the sniall optic nerves. Tins retina, shaken by 

 the approach of the vavs of light, can transmit the ii.ipression of 

 them to the brain. But t!ie few rays whicli reac'a the smooth 

 eyes, either on account of the sometimes uiifavouraljle disposition 

 of the cornea, or on account of tlieir smalliies-^, -vould have been 

 fctill diminished, if there had beeii under the ntTx es a blackness 

 and opaque coating a«. observed in the coinpourd eyes. Thus, 

 in the smooth eves this coating is most generally of a clear co- 

 lour, or even sometimes completely white, ^^'e may therefore 

 presume that the blackish tunic of the compound eyes is de- 

 stined, by absorbing a part of the rays of light, to diminish the 

 sensibility of the optic nerves, whUe the almost colourless var- 

 nish of the smooth eyes, far from absorbing the lumiiious rays, 

 sends them back, and thus increases the excitability of the retina 

 formed hy the spreading of the small optic nerves. It seems at 

 least that it is in tins respect that the smooth eyes may be use- 

 ful to vision ; for their smallness is so great, that without tliis 

 arrangement thev never could procure sensations very distinct. 



Besides, the insects wiiich have smooth eyes only,have a greater 

 number of them ; and as their life is generally less active than 

 that of the sjiecies v/hich possess two kinds of eyes, we may pre- 

 sume that it was not necessary to tliem to ^)erceive objects at 

 distances so greit. We observe also that insects which have 

 smooth eyes only, keep almost constantly in dark places, or at 

 least do not go great distances, being also deprived of the organs 

 adapted for fiying. The afjlera', and the numerous class of the 

 larvae of the lep'idoptercp, furnish a striking proof of this. 



All the experiments which I made to determine to what ex- 

 tent the smooth eyes may be useful to the insects which present 



the 



