LIGHT 235 



12. Lay a triangular prism over and parallel to a pencil mark. 

 How many marks appear? Explain. 



13. Why do we not see rainbows near noon? 



14. In problems 6 and 7 of this list why may we neglect the 

 speed of light? 



The Sight. - - The eyes are the very complex external 

 organs in which the fibers of the optic nerve terminate, and 

 by means of which sight impulses are produced. The 

 optic nerve is stimulated by light ; but the structure of 

 the eye is such that there is not a flood of light falling 

 upon the termini of the nerve fibers but a perfect image 

 of the lighted objects in the field of vision. 



Protection of the Eyes. - - The eyeballs are two nearly 

 spherical bodies. They are admirably protected by being 

 set into deep sockets. These sockets are lined with 

 fatty tissue in such a way as to protect the eyes from 

 shocks. The eyeballs are further protected by movable 

 folds of the skin called eyelids, and the eyebrows. The 

 eyelids are fringed by a row of stiff hairs which keep out 

 dust particles and help to shade the eye from irritating 

 lights. They are lined with a mucous membrane called 

 the conjunctiva and fit the front of the eyeball perfectly. 

 Friction between the eyelids and the eyeball is prevented 

 by mucous secreted by the conjunctiva and tears or 

 lachrymal fluid secreted by the lachrymal gland. Ex- 

 cess lachrymal fluid is drained into the nose through a 

 small duct called the lachrymal duct. The flow of the 

 lachrymal fluid is increased by irritations of the covering 

 of the eye or by strong emotion. In such cases the lach- 

 rymal duct is unable to drain the fluid off fast enough, 

 and tears overflow on to the face. 



The movements of each eye are controlled by six muscles 

 so arranged that the eyeball may be turned in any direction. 



