and smooth or simple Eyes of Insects. 277 



aequently we are not warranted in establishing between these two 

 organs any comparison. All that we can infer is, that it is as 

 necessary for insects to see external objects, as it is for animals 

 of a higher order. The carnivorous insect ought to have the 

 facultv of perceiving its food at a certain distance, as well as the 

 bird of prev, which suspended in the air would, perhaps, be con- 

 demned to die of hunger, if the acuteness of its vision did not 

 ensure its existence. 



It must however be observed, that if we only took into the 

 account the great convexity of tlie cornea of the eyes of insects, 

 we might Avell regard these animals as myopes ; for we know 

 that the more convex the crystalline is in the eyes of the verte- 

 bral animals, the more the spernes which present this arrange- 

 ment are myopes, p^irticularlv if the corneals very salient, if the 

 humours are verv dense, and finally, if tlie eye is too much sunk. 

 The mvopism tlicn depends on the rays being united before 

 reaching tlie retina: this inconvenience may therefore be remedied 

 bv the help of concave glasses, which retard the union of the 

 luminous rays. But the insects which present a very prominent 

 and convex cornea, are they also myopes as a consequence of 

 this arrangement, as Avould be the case with animals in which 

 the inrages fall upon the retina, after having traversed humours 

 of different density ? Although it is very difficult to solve so in- 

 tricate a question, it would nevertheless appear that the con- 

 vexity and protuberance of tlie cornea ought not to render the 

 insects myopes, since the luminous rays have not to unite rather 

 at one point than at ai'-othcr in order that the effects of vision 

 may be obtained, the latter Ijeing effected not because an image 

 is painted on the retina, but by the direct impression of the lu- 

 minous rays on the optic nerves. The convexity of the cornea 

 being favouralile to the collection of the rays, and hindering their 

 dispersion, this form was perhaps the most convenient in the 

 structure of the eyes of insects. It was at least the best adapted 

 for giving them keen sensations, since it tends to make the rays 

 of light fall more perpendicularly. This form is also the most 

 eominon in this order of auin;ials ; and even a considerable num- 

 ber of in>ects which perceive objects very far off, present their 

 cornea very convex and prominent, such as the papillons and 

 the cichidelce. It may al'^o be observed, that the concavity of 

 the cornea, by allowing only a small number of rays to enter the 

 eye, ought on that very account to procure less lively and 

 feebler sensations. Thus, we only observe this arrangement in 

 a small imniber of insects, and merely in the tolerably smooth 

 eyes ; but in these species, the compound eyes and the lateral 

 simple eyes may supply the want of the concave eye. 



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