X-RAYS: A SKETCH OF THEIR HISTORY. 
—--— — - 
other gases, as-does ultra-violet light; they have marked physiological effects . 
analogous to “sun-burn.” But they differ so markedly from ordinary light 
in their extraordinary power of penetrating opaque materials, and likewise 
‘in the impossibility of causing them to undergo reflection, refraction, or 
interference, that Réntgen himself left their nature an open question and 
signalised this mystery in the name he gave them. 
~ Although continual accretions took place to the knowledge gained by 
Rontgen (and in particular the discoveries of Barkla that to each element 
used as a targeb a characteristic X-radiation pertained and that the 
rays, like light-waves, were susceptible of polarisation, must be mentioned), 
yet the real character of these radiations remained unproyed, though not 
unguessed, until the year 1912. In that year Laue, Professor of Physics in 
the University of Munich, who in common perhaps with the majority of 
German physicists at the time held to the view that X-radiation was merely 
light of exceedingly short wave-length, conceived the brilliant idea of using 
the regularly-spaced arrangement of atoms in a crystal as a diffraction 
grating for X-rays, arguing that the relation of the atomic inter-spaces 
to their wave-length might be expected to be similar to that borne 
by the spaces between the lines of an ordinary grating to the wave- 
length of light. Experiments completely verified this conjecture. (V. Fig. 3.) - 
ElGaes ot Dire tontcla bearmmete cerien bmpectare 
through Sa plate. The central spot marks the 
incidence of the direct beam, the three pairs of spots are ,” 
its diffraction images. (From the original by Laue, 
_ Friedrich, and Knipping) = = 
Laue himself. used the crystal as a transmission-grating. We aby Bragg ‘28 
showed that it could be used to better purpose as a reflection-grating, 
and-in the hands of the Braggs (father and son), or Moseley, and of other — 
_ investigators. this new method of investigation speedily produced a vast in- 
crease in our knowledge of the X-rays themselves on the one hand, and on 
the other hand, of the arrangement of atoms in crystals. Barkla’s 
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