. ^â i.^u^'* NICKEI. can use. Taking as unit of length the Angstrom unit, which is one hundred-millionth of a centimetre, we cannot see detail finer than about 3,000 Angstrom units. The distances separating the molecules in a crystal we know to be of the order of two or three Angstrom units, cind so they are far beyond the possibihty of microscopic investigation. The discovery which opened the way towards finding how a crystal was built was made in 1912 by Laue. He was inspired to try the effect of passing a beam of X-rays through a crystal. It had already been suspected that X-rays
. ^â i.^u^'* NICKEI. can use. Taking as unit of length the Angstrom unit, which is one hundred-millionth of a centimetre, we cannot see detail finer than about 3,000 Angstrom units. The distances separating the molecules in a crystal we know to be of the order of two or three Angstrom units, cind so they are far beyond the possibihty of microscopic investigation. The discovery which opened the way towards finding how a crystal was built was made in 1912 by Laue. He was inspired to try the effect of passing a beam of X-rays through a crystal. It had already been suspected that X-rays consisted of electromagnetic waves of exactly the same nature as heat and light radiations, but with a wave-length which was one ten-thousandth that of visible light. Laue reaUsed that such waves are smaller than the distances between diffracted beam. In other directions the waves are not in phase, and the general effect is zero. The analysis of a spectrum by a grating depends on this principle. Laue reahsed that a crystal provides a diffraction grating where the regular arrangement of molecules takes the place of the hnes ruled on a metal grating, and where the distance between the molecules is of the right dimensions to diffract the very short X-rays. The results which he obtained when he passed a beam of X-rays through a crystal are shown in Fig. 2. In his apparatus a very narrow pencil of rays fell on a small slip of crystal, and a photographic plate was placed on the far side at right-angles to the rays. The large spot in the centre of the plate is produced by the direct beam of rays, the pattern of small spots
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