The classical work of Rydberg, and that of Kayser and Runge, dealt mainly with series lines in spectra which are developed in the flame or electric arc, or in vacuum tubes with discharges of moderate intensity. The lines to be discussed in the present communication are some of those which belong to the class of enhanced lines as defined by Lockyer; that is, they are relatively intensified in passing from arc to spark conditions. In considering these lines it is necessary to take account of the fact that their behaviour in the arc is different for different elements. At least three classes may be recognised:—(I.) Enhanced lines like the H and K lines of calcium, which are well developed in the ordinary arc; (II.) Lines which only appear with small intensities in the arc, such as the enhanced lines of iron; (III.) Lines which do not appear in the ordinary arc (except very locally near the poles), but are strongly developed under spark conditions, as in the case of the well-known magnesium line at λ 4481.