48 SUPPLEMENT ON 



in the middle than at the edge, and what is very extraordi- 

 nary, these lenses form the only apparently transparent part 

 of this complicated organ. The number of lenses varies con- 

 siderably in the different genera : seventeen tliousand, three 

 hundred and twenty-five have been counted in the eye of 

 a butterfly, but they are said to be much more numerous 

 in the beetle of Mr. Mac Leay's genus Dynastes. Leeu- 

 weenhoeck counted almost twelve thousand in the eye of the 

 dragon-fly. In the genus Xenops, however, these lenses do 

 not exceed fifty ; they vary in magnitude in the different 

 genera, and sometimes in the same eyes. 



Immediately behind the transparent lens and its socket, 

 is an opaque plaster, varnish or tunick, which varies consi- 

 derably in colour in the different species, and which fre- 

 quently forms in the eye itself dark spots and configura- 

 tions. This plaster covers the entire concave surface of the 

 whole aggregate cornea, and consequently the inner surface 

 of each particular lens, without leaving the slightest aper- 

 ture for the passage of light ; how therefore the image of the 

 object, after being refracted through the lenses, passes this 

 apparently opaque substance, remains to be explained : at 

 present, it seems most probable that we are deceived as 

 to the real opacity of this tunick. 



The object of the configurations formed by the differ- 

 ent colours, and probably l)y tlie different degrees of opacity 

 and transparency in the tunick of the eye, is not ascertained, 

 though it seems not improbable that it may be intended to 

 absorb a portion of the rays of light, and thereby diminish 

 the too great sensibility of the optic nerves : if this be so, 

 it is not perhaps the only instance in which Nature seems 

 to have had recourse to especial means to remedy her own 

 luxuriance. Fig. 5, shews the aggregate cornea of the eye 

 4 of Phaboinhyx luhricepeda^ with the coloured tunick alto- 

 gether i-emoved, and Fig. 6, the genei-al appearance of the 



