The instrument described was designed to give a high intensity of radiation at the photographic plate and to be capable of accurate angular measurements of crystal reflection. The Seeman form of spectrometer, in which a single slit is placed between the crystal (or ruled grating) and the photographic plate, was used together with a method of measuring angles on a single accurate circle. Both spectrometer and X-ray tube were placed in a common vacuum to allow the focal spot to be brought close to the crystal. The instrument has been used with a method similar to Rowland’s coincidence method to measure the wave-length of the AlKα
12
line relative to the fifth and sixth orders of the CuKα
12
doublet, using a sugar crystal. The result obtained, after correction for deviation from the Bragg law, is 8315 X.U. relative to 1539·23 X.U. for the copper doublet, the results being reproducible to 1 X. U.
Response of Barrier-Layer Cells to X-Rays of Long Wave-Length