THE EYES. 41 



number of minute eyelets, not unlike the six-sided 

 facettes of crystals. They are always immoveable, 

 differing in this from the eyes of larger animals. 

 Each of the little eyelets or facettes is very similar 

 in structure to one of the simple eyes already de- 

 scribed. According to Professor Miiller, of Bonn, the 

 composite eye of the dragon fly may be divided into 

 two parts ; one above and behind of a reddish colour, 

 with the eyelets twice as broad as those of the other 

 in front and below, which is greyish. 



When the whole composite eye is cut into, we find, 

 1. the outer transparent membrane (a) ; 2. the larger 

 or clammy-coloured matter (6); 3. abroad belt, orange- 

 coloured before, and black in front (c) ; 4. a second 

 belt within the first, deep black (d) ; 5. the ganglion 

 of the nerve, which, when slightly pressed, is seen to 

 be composed of rays of fibres, or threadlets (e), one of 

 which probably passes to each eyelet. 



. The cellular tissue, and the clammy-coloured sub- 

 stance, are found wanting in some species of night 

 insects ; and, according to Treviranus, there is in the 

 cockroach, (a night insect), behind the outer mem- 

 brane, a mass of a dark violet colour, composed of 

 numerous little pyramids, upon which the nerve is 

 spread in the form of fibres. 



