IN a recent communication, A. V. Baez1 predicts the practical resolution obtainable by diffraction microscopy using X-rays. His prediction is based on the required conditions for making the hologram. Similar derivations were described by us at the International Conference on Electron Microscopy in Paris, September 1950, though unfortunately we are still awaiting publication of the proceedings of this conference. Our derivation relates to the application of the method to electron irradiation where lenses can be used for producing intense coherent sources or to magnify the diffraction image (hologram). Our results are immediately applicable to the X-ray case and lead to the same conclusions as reached by Baez.