vii.] THALEN'S RESEARCHES. 91 



his doing so, and at his request Thalen, his associate, undertook 

 their preparation. 1 



Thalen's method of work was to compare the metallic lines 

 with the solar lines, and his wave-lengths were then taken from 

 Angstrom's Spectre Normal. 



He, like Kirchhoff, employed a large Kuhmkorff coil and 

 Ley den jar, and took the spark between electrodes of the metal 

 under observation, or of platinum or aluminium moistened 

 with solutions of salts of the various metals. The salts em- 

 ployed were usually the chlorides, on account of their ready 

 volatility. Occasionally, instead of the spark, an arc produced 

 by fifty cells was used. 



The dispersion generally employed was afforded by a single 

 prism of bisulphide of carbon, and was magnified by a very 

 powerful telescope. Sometimes a second similar prism was 

 added, and on rare occasions these were replaced by six prisms 

 of dense flint glass. Occasionally also, when low dispersion 

 was required, a single prism of flint glass was used. 



In some cases when the metallic lines were very faint, the 

 direct method of solar comparison could not be applied, because 

 the glare of the sunlight, even when reduced by a diaphragm, 

 was so great by comparison that the feeble metallic lines could 

 not be discerned. Another method had, therefore, to be re- 

 sorted to. Instead of producing the solar and metallic spectra 

 side by side, they were superposed. The sunlight being then 

 cut off, a pointer was brought into exact coincidence with the 

 line observed, and then the sunlight was again allowed to enter, 

 and the position of the pointer among the Fraunhofer lines gave 

 the position of the metallic line. 



Thalen gives only the strong lines of the metals in his maps 

 because he deemed it impossible to prove that the feeble lines 

 were not due to some other element present as an impurity, 



1 Memoir c sur la Determination des Longueurs cCOndt des Raics Mctalliqucs. 

 Upsala, 1868. 



