164 EXPERIMENTAL GENERAL SCIENCE 



138. The Eye and Color. Color, as we have seen, is really 

 a sensation set up in the retina by light of a certain wave 

 length and from the retina transmitted to the brain. There 

 seem to be three sets of these nerve endings sensitive to red, 

 green and blue respectively. When any of these are defective 

 or refuse to carry their proper sensations, the eye will be color- 

 blind. A person color-blind for red will have difficulty in 

 distinguishing a red apple from the green leaves by color alone. 

 Yellow and blue are the colors most easily distinguished by 

 the color-blind. The nerves which carry color sensations are 

 easily tired and, if overtaxed, may fail to report accurately for 

 a time. When one looks intently at a red object for a few 

 seconds and then at a white wall or curtain, he will see the 

 image of the object in green, since the tired nerves do not now 

 respond to that color in ordinary light and only the comple- 

 mentary colors are reported to the brain. 



Practical Exercises 



1. How many times is a beam of light turned in passing through a 

 prism ? 



2. Why do openings into dark places such as cellars, always appear 

 black? 



3. Why does the pupil of the eye appear black? 



4. Why is the interior of a camera always painted black ? 



5. Put a strong solution of soap and water in a flask and look through 

 it toward the light. What is its general color? 



6. Look at the flask by reflected light. What is the general color? 



7. How did the particles of soap in the water produce the effects 

 noted in the foregoing experiments? 



