Whereas extinction effects would be expected to decrease the x‐ray transmission through a crystal when it is set at a reflecting angle, it is shown experimentally that in many cases the transmission increases at the Bragg angle.This is believed to be due to the fact that the decrease in the effective absorption coefficient overshadows the losses due to reflection.A reconsideration of Darwin's presentation of extinction allowing for variable &mgr; shows that the transmission may increase or decrease depending on the values of &mgr; andp.Thus the earlier observations of Bragg and co‐workers are reconciled with the observations reported here.The importance, to crystal structure workers, of determining how &mgr; varies with direction in periodic media is obvious.