EVOLUTION THE MASTER-KEY 



with its limitations and wielded a new instrument, 

 the spectroscope. In its simplest form this is sim- 

 ply a prism, which spreads out a beam of white 

 light into its components, the colors of the spec- 

 trum. This was the famous experiment by which 

 Newton proved the composite nature of white 

 light. 



Now the spectroscope gives different results 

 according as it is placed in the path of light from 

 a glowing gas or light from a solid body. The 

 spectrum of sunlight is continuous, consisting of 

 bands of colors which shade off into one another. 

 The spectrum of a true gas, on the other hand, 

 consists of a series of bright lines separated by 

 dark intervals, and is known as a discontinuous 

 spectrum. It was shown by Sir William Huggins 

 that the spectrum of a young or chaotic nebula is 

 discontinuous, which is a proof that these nebulae 

 are not distant star-clusters, but are what they 

 appear to be, clouds of gas, often many times 

 greater in extent than the diameter of the solar 

 system. But Huggins applied his spectroscope 

 to the light from a spiral nebula, with the most 

 significant result that its spectrum was found 

 to be continuous. The denser patches in the 

 spiral nebulas, therefore, indicate places where the 

 nebula is beginning to solidify, where planets are 

 beginning to be formed. I say planets, taking the 

 solar system as a type, but we must remember 

 that the nebula from which our system is formed 

 was a comparatively small one. 

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