For material stratified in parallel planes in local thermodynamical equilibrium we have Milne's well-known result thatwhere πF is the constant net integrated flux of radiation, τ is the optical depth and B is the “ergiebigkeit” which is related to the temperature T by the relationσ being the Stefan-Boltzmann constant. If we define the effective temperature by the relationwe have from (1)where T1 is the temperature ar τ = 1. In this note we establish a similar result for extended photospheres where the curvature of the outer layers is properly taken into account.