LIGHT 239 



Most objects are non-luminous, every point sending out 

 rays of reflected light. The rays that enter the eye are 

 focused in proper scale by the cornea and crystalline lens. 

 If for any reason these rays are not properly focused, 

 defective vision results. For example, " near sight " 

 results when the focus is too short and the rays meet and 

 cross in front of the retina. ." Near sight " may be cor- 

 rected by the use of a concave lens in front of the eye. 

 " Far sight " is due to the focus falling at a point behind 

 the retina. Convex lenses are used to remedy this de- 

 fect. Astigmatism is due to imperfections in the cur- 

 vature of the cornea. The light which falls on some 

 particular spot of the cornea is not properly refracted. 

 A good oculist usually can correct these defects in vision 

 and should be consulted whenever they appear. 



QUESTIONS 



1. How are the eyes protected? 



2. Of what value are tears? 



3. Where are the lachrymal glands situated? 



4. Through what eye media does the light pass on its way to 

 the retina? 



5. Why is the inside of a camera black? 



6. What coat of the eye may be compared to the black interior 

 of the camera? 



7. To what is the color of the eye due? 



8. What is the use of the vitreous humor of the eye? 



9. What is meant by the power of " accommodation " as ap- 

 plied to the eye? 



10. What causes the iris to dilate and contract? 



11. Why can we not see the spokes of a rapidly moving wheel? 



12. The film of a moving-picture machine is really standing still 

 when the picture is thrown on the screen. Why does it seem to 

 be in motion? 



13. Name some cautions to be observed in the care of the eyes. 



14. What is " color blindness"? 



