An experiment was carried out on the intensity distributions resulting from the superposition of the images formed in the direction of each resultant order to the diffraction and optical interference pattern of a plane amplitude grating viewing a set of slits lying in a parallel plane. The experiment demonstrated the causes of blurring of the moire´ fringes formed by two gratings separated by an air gap. From experiments conducted with pairs of identical amplitude gratings the blurring of spatial moire´ fringes is demonstrated by microdensity tracings. It was found that the optical effects did not affect the interfringe spacing of the fringes.