At Cambridge University George Liveing and James Dewar set out to systematically measure spectra of elements from groups I, II and III in visible light and longer wave ultraviolet that would transmit through air. They noticed that lines for sodium were alternating sharp and diffuse. They were the first to use the term "diffuse" 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 first subordinate series for the diffuse series.
Arno Bergmann found a fourth series in infrared in 1907, and this became known as Bergmann Series or fundamental series.
Heinrich Kayser, Carl Runge and Johannes Rydberg found mathematical relations between the wave numbers of emission lines of the alkali metals.
Friedrich Hund introduced the s, p, d, f notation for subshells in atoms. Others followed this use in the 1930s and the terminology has remained to this day.