388 COORDINATION AND SENSATION 



the same manner as from the lower lid. A large lens 

 may be used to good advantage in finding the irritating 

 substance. 



Strong Chemicals in the Eyes. Students in the lab- 

 oratory frequently, through accident, get strong chemicals, 

 as acids and bases, in the eyes. The first thing to do in 

 such cases is quickly and thoroughly to flood the eyes with 

 water. Any of the chemical which remains may then be 

 counteracted by the proper reagent, care being taken to 

 use a very dilute solution. To counteract an acid, use 

 sodium bicarbonate (cooking soda), and for bases use a 

 very dilute solution of acetic acid (vinegar). To guard 

 against getting the counteractive agent too strong for the 

 inflamed eye, it should first be tried on an eye that has not 

 been injured. 



Summary. The nervous impulses that cause the sen- 

 sation of sight are started by light waves falling upon a 

 sensitized nervous surface, called the retina. By means of 

 refractive agents, forming a part of the eyeball in front of 

 the retina, light from different objects is focused and made 

 to form images of the objects upon this surface. In this 

 way the light is made to stimulate a portion of the retina 

 corresponding to the form of the object. This, the image 

 method of stimulation, enables the mind to recognize objects 

 and to locate them in their various positions. While the 

 greater portion of the eyeball is concerned in the focusing of 

 light, the crystalline lens, operated by the ciliary muscle, 

 serves as the special instrument of accommodation. Muscles 

 attached to the eyeballs turn them in different directions, 

 and so adjust them with reference to each other that 

 double vision is avoided. 



Exercises. i. Under what conditions are light waves reflected, 

 refracted, and absorbed? 



