An instrument is described which provides an accurately monochromatised beam of X-rays by reflection at a crystal and permits the wave-length to be changed without moving either the X-ray tube or the measuring apparatus. This is effected by an automatic adjustment which moves a second crystal into the correct position and orientation to reflect the monochromatised beam along a fixed emergent direction. Twenty wave-lengths well distributed in the range 0·8Å.—4Å. are available, the time required to select any one being about one minute. The whole instrument is contained in a small metal box which is evacuated.

I am indebted to Professor A. M. Tyndall, F.R.S., for extending to me the facilities of his laboratory, and for his interest in this work.