584 



THE MICROSCOPE. 



Fig. 26I. 



organ, with its series of 



The structure of the eye is in all creatures a most 

 admirable piece of mechanism, in none more so than in 

 those of the insect tribe. The eyes differ in each species ; 

 varying in number, situation, 

 figure, simplicity of con- 

 struction, and in colour. Fig. 

 260 represents a portion of 

 the eye of the common Fly, 

 drawn by the light of the sun 

 upon a prepared photographic 

 surface of wood ready for the 

 engraver. Fig. 261 repre- 

 sents a side view of the eye 

 when thrown down, showing 

 the compound nature of the 

 cylindrical tubes ; better seen in fig. 262. 



" On examining the head of an insect, we find a couple 

 of protuberances, more or less prominent, and situated 

 symmetrically one on each side. Their outline at the base 

 is for the most part oval, elliptical, circular, or truncated ; 

 while their curved surfaces are spherical, spheroidal, or 

 pyriform. These horny, round, and naked parts seem to 

 be the cornese of the eyes of insects ; at least, they are 

 with propriety so termed, from the analogy they bear to 

 those transparent tunics in the higher classes of animals. 

 They differ, however, from these ; for, when viewed by the 

 microscope, they display a large number of hexagonal 

 facets, which constitute the medium for the admission of 

 light to as many simple eyes. Under an ordinary lens, 

 and by reflected light, the entire surface of one cornea 

 presents a beautiful reticulation, like very fine wire gauze, 

 with a minute papilla, or at least a slight elevation, in the 

 centre of each mesh. These are resolved, however, by the 

 aid of a compound microscope, and with a power of from 

 80 to 100 diameters, into an almost incredible number 

 (when compared with the space they occupy) of minute, 

 regular, geometrical hexagons, well defined, and capable of 

 being computed with tolerable ease, their exceeding minute- 

 ness being taken into consideration. When viewed in this 

 way, the entire surface bears a resemblance to that which 

 might easily and artificially be produced by straining a 



