178 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



EXPT. 174. Procure a strip of wood about 12 x 1 x ^ inch, 

 and glue a glass prism upright at one end of it. Stick a pin 

 or a needle upright in the other end. Allow the needle to be 

 illuminated by the flame of a spirit lamp into which salt is 

 gradually allowed to fall. An orange-coloured image of the 

 needle is seen. Now illuminate the needle by a candle or gas 

 flame, taking care that the direct light from the candle does not 

 fall upon the face of the prism (Fig. 91). No longer is a single 



FIG. 91. To illustrate Experiment 174. 



image of the needle seen, but a complete band of colour from 

 red to blue. This is produced by an innumerable multitude 

 of images close together. 



A prism may thus be used, and is used, to analyse light. The 

 light of incandescent sodium vapour when observed through a 

 prism is characterised by the orange-coloured ray seen in the above 

 experiment with common salt, which contains sodium, and the 

 light emitted by other substances when burning are each dis- 

 tinguished by rays of a particular colour and position in the 

 spectrum. 



