X rays and crystal structure . acrystal, diffracted pencils would be formed, arrangedabout the primary beam in a regular pattern accord-ing to laws which he formulated. A photographicplate placed perpendicular to the primary rays andbehind the crystal would show a strong central spotwhere the primary rays struck it, and other spotsarranged in regular fashion round the central spot inthe places struck by the diffracted pencils. Theexperiment was carried out by Messrs. Friedrichand Knipping in the spring of 1912, and was abrilliant success from the first. Since then theauthors have pursued their
X rays and crystal structure . acrystal, diffracted pencils would be formed, arrangedabout the primary beam in a regular pattern accord-ing to laws which he formulated. A photographicplate placed perpendicular to the primary rays andbehind the crystal would show a strong central spotwhere the primary rays struck it, and other spotsarranged in regular fashion round the central spot inthe places struck by the diffracted pencils. Theexperiment was carried out by Messrs. Friedrichand Knipping in the spring of 1912, and was abrilliant success from the first. Since then theauthors have pursued their investigations vigorously,and their diagrams have attained the most admirableclearness. Examples are given in Plate I. Thebeautiful geometrical arrangement of the pattern isin reality a manifestation of the regularity of crystalstructure. We shall not discuss now the mathematical inves-tigation of the theory of the space-grating. Experi-ence has shown that there is an exceedingly simple Plate I. y ?- • BERNL Facing />itge 6. INTRODUCTORY 7 method of attacking the question which differs inform, though of necessity not in essence, from theorioinal method of Laue. The newer method leadsalso to a simple and useful mode of experimentalprocedure. It will be convenient to follow it for thepresent. J CHAPTER II. DIFFRACTION OF WAVES. The appearance of the photographs obtained by Lauesuggests at once the action of interference. Generally,when X-rays fall on a body which scatters them, thescattering takes place in a continuous manner allround the body. In this case, however, the scatteringtakes place in certain directions only, and the scatteredrays are grouped into separate pencils which leavetheir impression on the photographic plate in a seriesof isolated spots as shown in Plate I. The arrange-ment of these spots shows, both by its regularityand by the form which the regularity takes, thatthe effect is intimately connected with the crystalstructure. It must
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