Contribution functions and the depths of formation of spectral lines

A rigorous expression is derived for the contribution function to the spectral line depression in a stellar atmosphere; this, in effect, gives the contribution of the different atmospheric layers to the formation of the line depression. This contribution function is the solution of the transfer equation for the line depression. It is the only appropriate distribution function for the computation of the depths of formation of spectral lines. It indicates, in contrast to some earlier contribution functions, that a faint spectral line is not necessarily formed in the same layers as the continuum. The response function of the line depression to a given perturbation is briefly discussed.