A theoretical expression has been derived for the intensity distribution in the diffrimoscopic image for a straight edge under partially coherent illumination. Computations have been made to see (a) the effect of the degree of coherence, (b) the effect of the width of the aperture, and (c) the effect of obstacle width. It has been shown that the dark fringe appears exactly in the position of the geometric image of the edge and that the fringe width is practically independent of the degree of coherence and of the obstacle width but is inversely proportional to the width of the aperture.