148 COINCIDENT DARK AND COLOURED LINES. PARTI. 



/'double yellow sodium line and the double dark line D of 

 the solar spectrum, though he was not aware to what it 

 * was due. This coincidence, observed by M. Kirchhoff 

 many years afterwards, was fully appreciated by him, 

 and became the foundation of one of the most brilliant 

 discoveries of modern times. During a systematic 

 comparison between the spectra of volatilized sub- 

 stances and the solar spectrum, he discovered a perfect 

 coincidence between Fraunhofer's dark lines and all 

 the bright and coloured lines on the spectra of the 

 volatilized substances, sodium, calcium, magnesium, 

 chromium, iron, and nickel. To these M. Angstrom 

 has added aluminium and manganese, and M. Pliicker 

 has very recently found that all the three bright lines 

 in the hydrogen spectrum are coincident with dark 

 solar lines, and that none of the potassium lines corre- 

 spond with any solar lines. 



Drawings have been made of Fraunhofer's spectrum 

 placed above the spectra of the principal metals and 

 metallic salts, in which the coincidence of the bright 

 and dark lines is shown from the line A in the extreme 

 red to the line G in the indigo, and as the length of an 

 undulation of the extreme violet light of the solar 

 spectrum is the 1>(x y >000 of an inch, and the length of an 

 undulation of the extreme red is the ,-^QOO of an inch, 

 the length of the undulations of the intermediate rays 

 can be computed by the undulatory theory of light. The 

 length of the waves corresponding to Fraunhofer's seven 

 principal lines and many of the intermediate ones have 

 been computed, so that when a bright or coloured line 

 is coincident with any of these, the length of its waves 

 is at once known. There are other tables of Fraun- 

 hofer's lines, and the coincident bright ones in which 

 each dark line is marked by its own number, as the two 

 principal lines in the double line D, which are expressed 

 by the numbers 1002-8 and 1006-8, and so with the 



