The second volume of this work, the first of the more detailed volumes planned to follow the general account of the subject given in Vol. I, is intended to provide an outline of the general optical principles underlying the diffraction of X-rays by matter, which may serve as a foundation on which to base subsequent discussions of actual methods and results. Accordingly, reference has been made to experiments only where these illustrate principles, and all details of actual techniques, and of their application to specific problems, have been considered as lying beyond the scope of the volume.