ATMOSPHERE AND LIGHT 127 



The moisture in the air may form into little droplets 

 high above the earth's surface, making clouds, or these 

 droplets may be near the surface of the earth. In this 

 case we name the moisture fog. If it collects on objects 

 attached to the surface we call it dew. 



FOG. 

 A low cloud formed near the surface of the earth. 



By determining the dew point as was done in Experi- 

 ment 73 and comparing this with tables which have been 

 prepared by meteorologists from many observations the 

 relative humidity can be readily determined. An instru- 

 ment for determining the humidity of the air is called a 

 hygrometer (Fig. 62). 



63. Effect of Atmospheric Conditions on Light. Experiment 



75. Allow sunlight to pass through a glass prism and fall upon a 

 white wall or piece of paper. How has the white sunlight been 

 affected ? Where did the colors come from ? In what order are the 

 colors arranged ? This group of colors into which a prism separates 

 white light is called the spectrum. 



If the light that comes from the sun is passed through a 

 glass prism, as in Experiment 75, it will be seen to be 



