Since writing the above, I have seen an account of experiments in relation to diffraction of X-rays, presented to the French Academy by MM. L. Calmette and G. T. Huillier, in which the authors claim to have obtained evidence that diffraction occurs. The following translation of MM. Calmette and Huillier’s paper is taken from the _Electrical Engineer_, N.Y., for July 22, 1896. “We have the honor of submitting to the Academy some photographic proofs obtained with the Röntgen rays by means of the following arrangement.” “Very near the Crookes tube there is a screen “E” (_diagram omitted_), of brass, perforated by a slit, the width of which has rarely reached a half mm. A second metal screen, E´, is formed of a plate provided with two slits or pierced with a window in which is fixed a metal rod of 1 mm. in diameter. This screen is placed at the distance, _a_, behind the former. Lastly, a photographic plate, enfolded in two leaves of black paper, is placed at the distance, _b_, behind the second screen, E´.” “The following table indicates, for each proof, what is the screen E´ used, and the value of _a_ and _b_ + _a_: E´. No. _a_ _b_ + Cm. _a_ Cm. 1. Rod of 1 mm. in 5 19.5 diameter 3. Rod of 1 mm. in 5.5 20 diameter 5. Rod of 1 mm. in 8.9 30 diameter 7. Two narrow slits, ? ? separated by a cylindrical rod of 1 mm. in diameter “On the proofs 1, 3, 5 the shadow thrown by the metallic rod is bordered on each side by a light band which shows a maximum of intensity. Within this shade we observe a zone less dark, which seems to indicate that the Röntgen rays penetrate into the geometrical shadow. Lastly, in proofs 3 and 5 we see, in like manner, a maximum of intensity along the margins of the window in which the rod is placed.” “In the proof No. 7 we perceive, in the middle of the two white bands, a fine dark ray, while in the shadow of the rod which separates the two slits there is seen a light ray.” “If we compare these results with those obtained with light in the same conditions, the slit being relatively wide and the intensity weak, it seems difficult not to ascribe them to the diffraction of the Röntgen rays.”