It is shown that in a parallel-plate free-air chamber the field distortion due to the top and bottom of the chamber case tends to compensate for the field distortion produced by the end walls of the case. The existence of such a compensatory effect would account for the fact that whereas a small positive correction factor for field distortion is applicable to the N.P.L. medium-energy chamber, no such correction factor is applicable to the corresponding N.B.S. chamber.