A mechanism for the production of light and dark contrasts in radiatively controlled lines.

It is shown that light and dark contrasts observed in the radiation of strong lines may arise from lateral differences in the local value of the scattering term alone, independently of the local values of temperature and pressure. Mechanisms for the production of such differences are suggested, and regions are pointed out in which they occur as 'features.' From numerical examples presented, it is seen that the effect of such features on the emergent radiation field is determined by their position with respect to the shape of the source function.