FIG. 217. PROFESSOR KIRCHOFF. 



CHAPTER XVIII. 



PHYSICS OF THE NINETEENTH CENTURY SPECTROSCOPY. 



THE starting-point of the history of our present subject may be 

 found in the memorable experiment in which Newton resolved 

 ordinary sunlight into its constituent coloured rays. The experiment 

 which belongs to the seventeenth century, has been described in a 

 previous chapter (page 217), and to this the reader is recommended 

 to refer. The next step in the course of discovery was not taken until 

 the opening of the present century, whenWollaston, instead of allowing 

 the light to enter the dark chamber through a round hole, made use 

 of a narrow slit parallel to which the refracting edge of the prism was 

 placed, as shown in Fig. 218. Newton had, in fact, viewed, not one 

 spectrum, but an indefinite series of spectra partially overlapping but 

 not coinciding with each other. It will be observed that as an ex- 

 tremely narrow beam of light, such as will pass through a slit not more 



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