THE LIII absorption edges of aluminium and magnesium have been measured. Since the continuous X-ray spectrum in this region (above 100 A.) is very feeble, I have used optical spark spectra from elements giving spectra rich in lines. As source for the spectra reproduced in Fig. 1 a spark between copper electrodes was used. The apparatus was a concave grating spectrograph with a glass grating, ruled at this laboratory, of radius 1 m., 288 lines per mm.
Kruger and Copper, Phys. Rev., 44, 826; 1933.