They were the first to use the term "sharp" for the lines. They classified alkali metal spectral lines into sharp and diffuse categories. In 1890 the lines that also appeared in the absorption spectrum were termed the principal series. Rydberg continued the use of sharp and diffuse for the other lines, whereas Kayser and Runge preferred to use the term second subordinate series for the sharp series.

Arno Bergmann found a fourth series in infrared in 1907, and this became known as Bergmann Series or fundamental series.

In 1896

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