138 feature's dfoiracles* 



UM be thankful for the blue sky and golden 

 sunsets. 



" But," you ask, " why is the sky blue? " 

 In one of the chapters on the subject of 

 light in Vol. II. the properties of soap bubbles 

 are discussed. It is shown that when a film 

 is stretched across the mouth of a tumbler 

 held in a position so that the film is perpen- 

 dicular, by the action of gravity (the moisture 

 constantly falling to the lower part of the film) 

 it will continually grow thinner, and hori- 

 zontal bands of color will appear upon it, 

 first red, then followed by the other colors of 

 the solar spectrum, ending with violet. 



It is also stated that every color of light has 

 a definite wave length. Where a band of blue 

 color appears upon the film we know that its 

 thickness is right for the wave length of that 

 particular color which is reflected from the 

 back of the film to the eye. If we could con- 

 ceive the blue vault of the heavens to be half 

 a sphere of a soap bubble, the color that the 

 sky would appear to us (if the light could be 

 thrown upon it from beneath) would be deter- 

 mined by the thickness of this film. If the 

 film was 1-156,000 of an inch the sky would be 

 red instead of blue. To reflect the other 

 colors the film would have to grow thinner for 

 each color, in the progression from red to 

 violet. The color of the sky is determined by 

 a light-reflection from minute globules of 



