S INORGANIC EVOLUTION. [CHAP. 



But we are not limited to flame temperatures ; substances in a state 

 of gas or vapour may be made to glow by electricity. At these higher 

 temperatures very complicated spectra are produced, and again the 

 spectrum is special to each chemical substance experimented on ; the 

 images of the needle (or slit), occupying different positions along the 

 spectrum according to the nature of the source of light. 



Fig. 5 gives us a laboratory prism spectroscope of small disper- 

 sion ; with the more complicated spectra the phenomena are often 

 better seen if more than one prism is employed. Fig. 6 shows an 

 instrument in which four prisms are used. 



FIG. 6. Steinheil spectroscope with four prisms. 



It is in the case of the more complicated spectra that the wave- 

 length has to be specially considered from the point of view of denn- 

 ing the position of a line. It is not enough to say, as was said in the 

 case of the sodium line, that it is located in the orange. 



The lengths of the various light-waves are very small. The wave- 

 length of the sound-wave of the middle C of a piano is about 4 feet, 

 while the wave-length of yellow light as defined by that of a line very 

 accurately measured is -0005895 of a millimetre, that is 5895 ten- 

 millionths of a millimetre ; so that there are 43,088 waves in a British 

 inch. The unit of wave-length usually employed is the ten-millionth 

 of a millimetre. These wave-lengths get shorter as we pass from the 

 red to the violet. 



For accurate measures of the wave-lengths of the lines a grating 

 is employed as shown in Fig. 7. 



