88 SCIENCE PROGRESS 



The modern grating of Rowland used for the diffraction of 

 light-waves with its five thousand or more lines to the centi- 

 metre, ruled by a diamond on a glass plate with perfect 

 regularity, is rightly regarded as a triumph of mechanical art, 

 beyond which it is impossible for art to go. Yet something 

 ten thousand times finer had to be found before the X-rays 

 were to belie their name and take their right place in the 

 gamut of electromagnetic radiations, as far removed from 

 visible light-waves in the direction of smallness as these in 

 turn are in the opposite direction from those of the waves of 

 wireless telegraphy. 



For seventeen years after they were discovered by Rontgen, 

 the real nature of the X-rays was discussed and left undecided 

 until Dr. Laue, of the University of Zurich, in 191 2 conceived 

 the idea of employing a .crystal, with its marshalled ranks of 

 atoms, packed in regular order, many million to the linear 

 centimetre, to do for X-rays what the diffracting grating does 

 for visible light. The success of the attempt opened a new era 

 of activity, not for one but for many sciences, and the first 

 chapter of this new era in the science of crystallography is set 

 forth in these pages. 



It is fitting that in this work the pioneers in this country 

 should have been Prof. Bragg and his son. The y-rays of radio- 

 active bodies, and what are in effect artificially generated y-rays — 

 the X-rays of Rontgen — are old acquaintances of Prof. Bragg, 

 who turned to them, after his distinguished elucidation of the 

 problem of a-ray transmission, for fresh worlds to conquer. 

 For many years he laboured to establish a view of their nature 

 which is now mainly of historical interest, but it was the dis- 

 covery of Laue and his colleagues which finally directed the 

 inquiry into its present channels. 



The problems involved have a general resemblance to those 

 of the diffraction of light by a ruled grating, but are considerably 

 more complex, for, in the crystal, instead of lines ruled at equal 

 spaces in a plane, we have to deal with points or atoms 

 orientated in definite but unknown manner in three dimensions 

 of space. At the outset both the wave-lengths of the X-ray and 

 the distances between the diffracting points of the crystals were 

 quite unknown, but very soon both unknowns were determined 

 in absolute measure. A great simplification of the problem 

 was effected by the younger author at the outset. He showed 



