xvi.] PHYSICAL PECULIARITIES. 221 



Thirty-one metallic substances in all. It will be seen that 

 the photographic attack in this direction was of extreme 

 value . 



4. Physical Peculiarities of Lines. 



In the photographs of the arc spectra one of the first things 

 noticed 1 was, as already stated, that not unfrequently a very 

 thick line reversed itself, a circumstance which greatly facilitates 

 its comparison with confronted lines, since a thin dark line then 

 runs down the centre of the thicker bright one. The absorption 

 line does not always occupy the exact centre of the bright band. 

 Examples of this from the spectra of calcium and aluminium 

 were published at the time. 



These phenomena enabled us to draw a distinction between those 

 substances which give us winged and easily reversed lines in arc 

 spectra and those which do not. Among the former are sodium, 

 magnesium, calcium, strontium, silver, and some others. There is 

 no difficulty in seeing the reversal of winged lines in the case of all 

 spectra in which they exist, when the image of the arc is thrown 

 on the slit, and such lines lying in the region between K and Gr 

 were photographed in 1873. But although the various curious 

 phenomena which these reversals present are easily visible, it is 

 very difficult to photograph them in all their stages. 



Still with the long arc given by the Siemens' machine, several 

 of the various aspects put on during the process of reversal can 

 be caught. Chief among these phenomena are the various 

 thicknesses of the lines of reversal over the arc and poles, and 

 the appearance of the bright line without reversal in some 

 regions, and the reversal without the bright line in others. 



All the phenomena presented by the absorption of the D line 

 to the eye are in the photographs in duplicate. It may be useful, 

 perhaps, to state what phenomena are seen in the case of the D 



1 Phil. Tram. 1874, part 2, p. 805. 



