2 INORGANIC EVOLUTION. [CHAP. 



What nature accomplishes by a rain-drop we can do with a prism 

 or a grating. A prism is a piece of glass or other transparent mate- 

 rial through which the light is bent out of its course or refracted in the 

 process. A grating is a collection of wires, or scratches on glass or 

 metal, equidistant, very near together, and all parallel. When light 

 passes through, or is reflected by such a system, it is said to be diffracted, 

 and one result that we are concerned in is very similar to that of 

 passing light through a prism. 



It is rapidly becoming a familiar fact to many that when a ray of 

 white light is refracted by a prism or diffracted by a grating a band of 

 colour similar to a rainbow is produced, and that this effect follows 

 because white light is built up of light of every colour, each colour 

 having its own special length of wave and degree of refrangibility. 

 Our rainbow band is called a spectrum. 



Such a glass prism or grating is the fundamental part of the instru- 

 ment called the spectroscope, and the most complicated spectroscope 

 which we can imagine simply utilises the part which the prism or 

 grating plays in breaking up a beam of white light into its constituent 

 parts from the red to the violet. Between these colours we get that 

 string of orange, yellow, green, and blue which we are familiar with in 

 the rainbow. 



A Simple Spectroscope. 



For sixpence any of us may make for ourselves an instrument which 

 will serve many of the purposes of demonstrating some of the mar- 

 vellously fertile fields of knowledge which have been recently opened 

 up to us. From an optician we can buy a small prism for sixpence ; get 

 a piece of wood from 20 to 10 inches long (the distance of distinct 

 vision), 1 inch broad, and J an inch thick. On one end glue a cork 



Prism 

 Candle 



FiG. 1.-- Arrangement of candle, prisrn and eye. 



2 inches high, at the other end fasten, by melting the bottom, a stump 

 of a wax candle of such a height that the dark cone above the 



