The aim of this mathematical treatment is to prove that any distortion of the gaussian intensity distribution of the beam, which may be caused by the finite width of the slits, is small, provided that the slit widths are not large with respect to the source size and divergence. The widths of the slit A and of the slit B were the same, equal to 0.1 mm for the measurement of the horizontal source size and divergence, and equal to 0.025 mm in the vertical case. It turns out, as shown below, that the distortion of the phase space gaussian beam distribution is estimated to be not greater than 6% in the vertical case and is considerably smaller in the horizontal case. The procedure followed in the analysis of the experimental data is then justified and is a good approximation.