SENSE OF SIGHT. 



211 



opticians, they hover over, and when they see their prey, 

 dart down perpendicularly, in which direction, as we have 

 seen, there is no refraction. There is a curious fish in the 

 East India waters called chactodon, about eight inches long, 

 that appears to understand optics remarkably well. When 

 it sees a fly, sitting on the plants that grow in shallow water, 

 it swims within five or six feet, and then with the dexterity 

 of a practical marksman, ejects from its tubular mouth a 

 single drop of water which never fails to strike the fly into 

 the sea, where it soon becomes its prey. Dunglison states 

 that Hommel, the Dutch governor, put some of these fish 

 into a tub of water, and then pinned a fly on a stick within 

 their reach. He daily saw the fish shoot at the fly, and they 

 never failed to hit their mark. 



32. Short-sightedness. -This generally arises from too 

 great convexity of the cornea, or excessive density in the 

 structure of the crystalline lens ; either of which will cause 

 the visual rays from near objects, to converge to a focus, be- 

 fore they reach the retina. This is remedied by concave 

 glasses^ which, as we have seen, cause the rays to diverge, as 

 is represented in the following cut. 



Fig. 17. 



A. a short-sighted eye ; B. an arrow which it attempts to perceive, 

 but is prevented by the convergence of the passage of the visual rays 

 to foci, at C., before they reach the retina at D. E. the same eye. 



