90 



over a million light-waves ; ; and the corresponding width of 

 spectral line would be less than a thousandth part of that which 

 separates the sodium lines. 



"^ 



Fig 5. 



Figure 5 illustrates the arrangement of the apparatus as it 

 is actually used. An ordinary prism spectroscope gives a pre- 

 liminary analysis of the light from the source. This is neces- 

 sary because the spectra of most substances consist of numerous 

 lines. For example, the spectrum of mercury contains two yellow 

 lines, a very brilliant green line, and a less brilliant violet line, 

 so that if we pass all the light together into the interferometer, 

 we have a combmation of all four. It is usually better to sepa- 

 rate the various radiations before they enter the interferometer. 



