PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 335 



come. In 1905 Barkla tended to confirm this view by show- 

 ing that X-rays could be polarized to some extent. 



But their real nature seemed first to be demonstrated in 1912 

 by Laue, who considering that if X-rays were very short light 

 waves they would be capable of diffraction by very small-meshed 

 screens, and by such only, with rare imaginative insight per- 

 ceived in the structure of crystals a possible diffraction-screen. 

 He surmised, that is, that the regularly arranged molecules of 

 a crystal, the space-lattice, as Bravais called it, might affect a 

 beam of X-rays somewhat as, for example, a finely meshed silken 

 fabric affects a beam of light. Confirming his hypothesis by 

 mathematical analysis, he, with Friedrich and Knipping, passed 

 a fine stream of X-rays through a thin slab of crystal, behind 

 which at a proper distance he set a photographic plate. He found 

 upon the plate after development an orderly arrangement of 

 spots, forming to all appearance a well defined diffraction pattern. 

 The X-rays behaved in fact, exactly like short light-waves. 

 Laue's apparatus was ingeniously modified by W. Lawrence 

 Bragg (with his father, W. H. Bragg, among the most diligent 

 investigators of such phenomena) who utilizing the known prin- 

 ciple of multiple reflections from equally spaced transparent 

 screens, observed the reflection of the X-rays from the crystalline 

 structure, instead of their diffraction on passing through it. He 

 constructed on this principle an X-ray spectrometer by which 

 the Roentgen radiations may be examined almost as readily as 

 one can examine with the ordinary spectroscope the radiations 

 which we call light. The surface of the substance under ex- 

 amination need not be polished, indeed it may not bear any defi- 

 nite relation to the direction in which the X-rays are moving, 

 since the reflections take place, not from the surface, but from 

 the successive layers of molecules within the crystal itself, and 

 it is these layers which must be properly oriented or directed 

 with regard to the beam of rays. A train of waves will not be 

 appreciably reflected from such an arrangement unless the wave- 

 length bears a definite relation to the spacings of the layers of 

 molecules. The effective distance between the layers may be 

 altered at will by rotating the crystal so that the X-ray waves 



