Abstract
LONDON. Royal Society, March 18.—Sir William Crookes, president, in the chair.—Prof. W. H. Bragg: Bakerian Lecture: X-rays and crystalline structure. The atoms of crystal may be conceived-in various ways-as arranged in a series of parallel planes, each capable of reflecting a small fraction of an incident pencil of X-rays. If the spacing of the planes is d, the wavelength λ, and the angle between the rays and the planes is θ, and if the relation nλ = 2d sin θ is satisfied, where n is any integer, then the various reflected pencils are in the same phase and combine to give an obvious reflection of the X-rays. If this relation is not satisfied there is no reflection. The X-ray spectrometer is designed to measure the various values of? at which reflection occurs in a given case. The angle can easily be determined to a minute of arc. Given d we can compare the wave-lengths of different X-rays. Given λ we can compare the spacings of various sets of planes of the same or of different crystals. By certain considerations the experiments can be made absolute and not merely comparative. In this way the structures of several simple crystals have already been found, such as rock-salt, diamond, iron pyrites, and so on. The reflections for various values of n, the integer in the formula, or, as they may be called, the spectra of various orders, differ amongst themselves in a surprising way. The intensities in the case of the most important planes in Iceland spar have recently been determined and give very interesting results. In the case of two pairs of planes the spacing is the same but the arrangement of atoms is different. This gives an opportunity of comparing the effect of arrangement apart from spacing, and it appears that the intensity of the reflection in any order is proportional to the weight of the planes which contribute to that order. Again, there are three calcite planes for which the arrangement of the atoms is exactly the same, but they differ in their spacings. The relative intensities follow a rule which has already been stated, viz., that the intensity in a reflection at an angle? is inversely proportional to sin 2?, other things being the same. Rules of this kind are needed if the method is to be used in the examination of more complicated crystals. The physical meanings that may. be attached to these rules are of considerable interest.
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Societies and Academies . Nature 95, 109–111 (1915). https://doi.org/10.1038/095109b0
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DOI: https://doi.org/10.1038/095109b0