280 The Outline of Science 



12 



What the Blue "Sky" means 



We saw in a previous chapter how the spectroscope splits 

 up light-waves into their colours. But nature is constantly split- 

 ting the light into its different-lengthed waves, its colours. The 

 rainbow, where dense moisture in the air acts as a spectroscope, 

 is the most familiar example. A piece of mother-of-pearl, or 

 even a film of oil on the street or on water, has the same effect, 

 owing to the fine inequalities in its surface. The atmosphere all 

 day long is sorting out the waves. The blue "sky" overhead 

 means that the fine particles in the upper atmosphere catch the 

 shorter waves, the blue waves, and scatter them. We can make 

 a tubeful of blue sky in the laboratory at any time. The beauti- 

 ful pink-flush on the Alps at sunrise, the red glory that lingers in 

 the west at sunset, mean that, as the sun's rays must struggle 

 through denser masses of air when it is low on the horizon, the long 

 red waves are sifted out from the other shafts. 



Then there is the varied face of nature which, by absorbing 

 some waves and reflecting others, weaves its own beautiful robe 

 of colour. Here and there is a black patch, which absorbs all the 

 light. White surfaces reflect the whole of it. What is reflected 

 depends on the period of vibration of the electrons in the particu- 

 lar kind of matter. Generally, as the electrons receive the flood 

 of trillions of waves, they absorb either the long or the medium or 

 the short, and they give us the wonderful colour-scheme of na- 

 ture. In some cases the electrons continue to radiate long after 

 the sunlight has ceased to fall upon them. We get from them 

 "black" or invisible light, and we can take photographs by it. 

 Other bodies, like glass, vibrate in unison with the period of the 

 light- waves and let them stream through. 



Light without Heat 



There are substances "phosphorescent" things we call them 

 which give out a mysterious cold light of their own. It is one 



