Curves of growth for the two stars Feige 34 (left) and PG1544+488 (right), chosen as examples of low and high column densities. For each graph, the y-axis represents the ratio between the equivalent width, W_λ, and the wavelength, λ, of the line, and the x-axis the product of the column density N expressed in cm⁻², the strength of the transition (unit-less f-value), and λ in Å. The blue circles represent the equivalent widths of the O Ilines used to determine the column density of OI and the Doppler parameter, b, and the red star the equivalent width of the Ar I line used to determine the [Ar I/O I] abundance ratio.