214 PHYSIOLOGY. 



humour, an iris, a choroid coat, and a retina ; the pigment 

 running back around the lens, separating each cylindrical 

 compartment. They have no eye-lids, eye-lashes, or tears ; 

 but these are compensated for by the great hardness and in- 

 sensibility of the cornea. 



Fig. 18. 



Eye of the yellow beetle magnified, composed of 8,820 hexagonal 

 cylinders, the interior of each tube being round. 



Fig. 19. 



Eye of the phalaena or butterfly, magnified, consisting of 11,300 

 square sections. The eye of the mordella is similar, containing 25,088 

 prisms. 



38. Motions of the Eye. We have seen that the eye is a 

 perfect optical instrument, infinitely surpassing the boasted 

 specimens of human skill ; but without the power of motion, 

 of adapting it to the varied objects which surround us, we 

 should derive but a very small proportion of that gratification 

 and pleasure, of which it is now the source. We shall per- 

 ceive the great advantages we enjoy from this benevolent 

 provision, if we contemplate the movements of a telescope ; 

 how difficult it is to direct it to any object, so as to obtain a 

 view of it, and what complicated and cumbrous machinery 

 are required to effect it. But all the various motions of the 

 eye are produced by six little muscles ! 



