$ 399] Spectrum Analysis 387 



that light of a particular colour found at a particular part of 

 the spectrum has a definite wave-length. At the extreme 

 violet end of the spectrum, for example, the wave-length 

 is about fifteen millionths of an inch, at the red end it is 

 about twice as great \ from which it follows ( 283), from 

 the known velocity of light, that when we look at the red end 

 of a spectrum about 400 billion waves of light enter the eye 

 per second, and twice that number when we look at the 

 other end. Newton's experiment thus shews that a prism 

 sorts out light of a composite nature according to the wave- 

 length of the different kinds of light present. The same 

 thing can be done by substituting for the prism a so-called 

 diffraction-grating, and this is for many purposes super 

 seding the prism. In general it is necessary, to ensure 

 purity in the spectrum and to make it large enough, to 

 admit light through a narrow slit, and to use certain lenses 

 in combination with one or more prisms or a grating ; and 

 the arrangement is such that the spectrum is not thrown 

 on to a screen, but either viewed directly by the eye or 

 photographed. The whole apparatus is known as a spectro- 

 scope. 



The solar spectrum appeared to Newton as a continuous 

 band of colours ; but in 1802 William Hyde Wollaston 

 (1766-1828) observed certain dark lines running across the 

 spectrum, which he took to be the boundaries of the natural 

 colours. A few years later (1814-15) the great Munich 

 optician Joseph Fraunhofer (1787-1826) examined the sun's 

 spectrum much more carefully, and discovered about 600 

 such dark lines, the positions of 324 of which he mapped 

 (see fig. 97). These dark lines are accordingly known as 

 Fraunhofer lines : for purposes of identification Fraunhofer 

 attached certain letters of the alphabet to a few of the most 

 conspicuous ; the rest are now generally known by the wave- 

 length of the corresponding kind of light. 



It was also gradually discovered that dark bands could 

 be produced artificially in spectra by passing light through 

 various coloured substances ; and that, on the other hand, the 

 spectra of certain flames were crossed by various bright lines. 



Several attempts were made to explain and to connect 

 these various observations, but the first satisfactory and 

 tolerably complete explanation was given in 1859 by Gustav 



