On the use of first-order perturbation theory in interface shift problems

This letter makes the following remarks with regard to the conclusion that the classic first-order perturbation formula is incorrect for the internal interface perturbation problem: an interface shift problem can be treated as either a material property perturbation or as a volume perturbation; and in a perturbation problem in which the interface shift is great but the difference in material properties on either side of the interface is small, it is more accurate to use the classic first-order perturbation formula