Measurements were made of the diffraction patterns of circular apertures from one to eight wave‐lengths in diameter in the planes of the apertures and in the neighborhood of the apertures when a plane polarized electromagnetic wave was incident upon them. From Thomas Young's theory that the diffraction pattern is an interference pattern between the incident plane wave and wavelets from the edge of the aperture, the positions of maximum intensity have been predicted in the neighborhood of the aperture and the values of the intensities over the apertures checked with experiment.