IRIS OP THE EYE. 293 



Their object in their perpetual waltz is not so much amuse- 

 ment as food, which chiefly consists of the tiny insects which 

 fall into the water. Now, in order to enable it to see both 

 above and below the water, a peculiar structure is required. 

 Generally the insects possess one pair of compound eyes, each 

 group being set on the sides of the head. In the Gyrinus, 

 however, there are two sets of these eyes, one pair being on the 

 upper surface of the head, and the other on the lower surface. 

 Thus, while it can use the upper pair for seeing objects which 

 are out of the water, the lower pair of eyes, which are sub- 

 merged, act the part of the Water Telescope, and enable it to 

 see objects that are below the surface. Were it not for this 

 precaution, even the ripples which it makes by its own rapid 

 progress would prevent it from seeing. 



THE IRIS OF THE EYE. 



I HAVE often wondered, when contemplating the astonishing 

 mechanism by which the Iris of the Eye is able to contract or 

 enlarge the pupil according to the amount of light, whether any 



IBIS OP HUMAV EYE. COTTKRILL's LOCK. 



similar mechanism would be used in Art. As anatomists 

 know, the Iris is composed of two layers. One consists of 

 radiating fibres, which serve to enlarge the pupil, while 

 the other layer surrounds the latter, and by its elasticity 

 serves to contract it. As any one may see by looking in 

 a mirror and shifting the light, the pupil is perpetually 

 changing its diameter, but always retaining its circular 

 shape. A glance at the illustration will show the two layers, 

 and aid the reader in understanding the mode in which they 

 work. 



