The kinematical upper limit of the extinction-corrected mean thickness as w
ell as the corresponding integral width of the intensity profiles are estim
ated for spherical crystals. The new expression for the primary-extinction
correction is compared with expressions given recently in the literature as
well as with those widely used in crystal structure analysis. (C) 2000 Int
ernational Union of Crystallography Printed in Great Britain - all rights r
eserved.