A study of the intensity distribution of almost 6000 spectrum lines indicates that the number of lines with a given intensity is inversely proportional to the square root of that intensity. The law appears to apply to smaller samples of only a few hundred lines as well as to the complete set. The analysis uses data from an iron hollow cathode lamp and is principally based on lines of Fe I, Fe II, Ne I , Ne II, Ar I, Ar II and He I lines are also included, and the wavelength range is from 2.6 to 0.28 mu m. The results is of application both to the calculation of plasma emissivity and to line blanketing in stellar atmospheres.