256 IMPORTANCE OF OUR EYES 



white cardboard back and forth until a position is found where a sharp 

 image of the filament is seen on the screen. Move the light farther away. 

 What must you do with the lens to give a second sharp image on the 

 screen? Compare the two images. Move the lens towards the light 

 until an enlarged image results. Explain how this experiment illustrates 

 the use of a lens in the ordinary camera and also in the enlarging camera. 



What is color ? If you have ever watched waves com- 

 ing in to the shore, you may have noticed that sometimes 

 the distance beween two successive waves is quite great, 

 and sometimes it is much less. We have learned that 

 light is caused by waves of ether which come to us from the 

 sun or from other lighted objects. Only a very small 

 portion of all the ether waves affect our eyes, and give us 

 the sensation we call light. Ether waves producing light 

 differ in length, the longest being one thirty-three thou- 

 sandth of an inch in length, and the shortest that we can 

 see, one sixty-six thousandth of an inch in length. The 

 longest of these visible waves causes the sensation of red, 

 and the shortest gives the sensation violet, while all other 

 colors come between these. Color, then, is a property 

 of light waves, and each particular color is due either to 

 waves of some particular length, or to mixtures of waves 

 which produce different colors. 



How we see color. When all the different light waves 

 from the sun are mixed together, white light is pro- 

 duced, as we see when sunlight is reflected by white paper. 

 White substances are those which reflect all sunlight. 

 Black objects are those which absorb all of it. Any object 

 which absorbs all the light waves except the very long ones 

 is red, because it reflects to your eyes only the red waves. 

 In the same way, any object which absorbs all the waves 

 except the very short ones would look violet in color. 

 This seems very strange, but we can prove that light is 



