

CHAPTEK VII. 



THE WORK OF ANGSTROM AND THALEN. 



WE now pass on to the next step, the work actually carried 

 on or inspired by another eminent man no longer amongst us, 

 Angstrom. He took up very nearly the same work as Kirchhoff 

 did, and extended it in certain directions ; but he did the work 

 in a different way instrumentally. Instead of the electric spark, 

 he made use of the arc produced by the passage of a constant 

 current of electricity from a powerful battery, between poles of 

 carbon, the lower one of which served as a receptacle or crucible 

 for the metal or salt under examination. A form of lamp 

 commonly employed for this purpose is shown in Fig. 33. The 

 poles are kept at a constant height and constant distance from 

 each other as they burn away, by clockwork controlled by an 

 electro-magnet. 



This very powerful light enabled Angstrom to use very high 

 dispersion. He was not content with the kind of scale which 

 Kirchhoff had employed, a scale dependent on the construction 

 of his instrument. He wished to have a natural scale. He 

 therefore rejected prisms, and used a diffraction grating, devised, 

 as we have seen, by Fraunhofer. By means of this he obtained 

 what was called, and what is still called, a normal spectrum; 

 and having obtained this, he, like Kirchhoff, endeavoured to 

 determine the confidence, or want of coincidence, of metallic 

 lines with those visible in the spectrum of the sun. 



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