382 



Microscopical Essays.. 



optic nerve, that divides into fix brandies, which are conneclei 

 with each eye • each of thefe is accompanied with a pulmonary 

 tube. This celebrated obferver, in fpeaking lightly of the reti- 

 culated eyes of the moth of the coflus, which are in number 

 22,000, fays, that each is probably a telefcope, confining at 

 leaft of three lenfes. 



It is eafy to fhew with the eye of a fly, prepared as we have 

 already defcribed in page 142, chapter iv. and then placed before 

 the microfcope, tnat each of the portions of thefe reticular eyes 

 have the power of refracling the light, and forming an image of a 

 candle, or any other obje£l, at their refpeclive foci. Mr. Martin 

 affirms, that in a great number of obfervations he always found 

 thatfthe edge of the cornea of each eye was always thicker in the 

 middle than at the edges, and that this fwelling out, or increafe 

 of thicknefs, was equal on each fide, and exaclly oppofite, 

 amounting to a full proof that the cornea of an infe&'s eye is 

 really a fyftem of a great number of exceeding fmall double and 

 equally convex lenfes, of an hexagonal form, " and affording one 

 of the moft fixiking inftances of Divine geometry and optics.'' 



Fig. 3, Plate XVI. is a reprefentation of a fmall portion of the 

 cornea of the libellula, as feen by the microfcope. The fides of 

 the hexagons, in fome pofitions of the light, appear of a fine gold 

 •colour, and divided into three parallel lines. Fig. 4 is the fame 

 piece of it's natural fize. 



Fig. 5, Plate XVI. reprefents a piece of the cornea of a IcMer s 

 eye ; in thefe each of the eyes are fet as it were in a fquare frame. 

 Fig. 6, the fame piece not magnified. 



,. . r r',.' 4* [f^rs ,y *y fi Mr. 



