3 The measurements

The plates were analysed using the Leitz UWM3 measuring machine. All plates were measured twice in positions differing by $180^{\circ}$, in order to eliminate remnants of magnitude error in setting on the large image.

Following the method used by van Albada-van Dien (1977), the relative positions of the components were obtained directly by measuring the secondary image and the zeroth order of the primary, instead of averaging the first or second diffraction images. This is to avoid asymmetric diffraction pattern due to centering error of the grating. The first (bb) and second (cc) order of Sirius A were also measured, to find the contraction of the emulsion, C, and to check on the asymmetry.