Communicated by W .L . I. I n t r o d u c t io n The broadening of the X-ray diffraction lines which occurs when the crystals composing the specimen are smaller than about 10 5 cm. edge length is well known. Since the discovery of this phenomenon by Scherrer in 1920 a fair amount of work has been done on the determination of the particle size from the breadth of the lines. Scherrer (1920) , Bragg (1933) and Seljakow (1925) have calculated the diffraction broadening for crystals of cubic form

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... onging to the cubic system. For parallel monochromatic radiation and a point specimen, they agree upon a formula a ) P* tcoshx where /? = angular breadth of the line defined below, t -edge length of cubic crystal, A = wave-length of X-radiation, the Bragg angle. The values given for the constant C are0 9 4 , 0-89 and 0*92 respectively. The most rigid theoretical treatment of the problem is that by Laue (1926), who has discussed the case of crystals of general parallelepiped form belonging to any crystal system, i.e. a crystal of edge lengths al5 m2a 2, m3a 3, where the m fs are numbers and the vectors a t define t For crystals of cubic form belonging to the cubic system, Laue's formula reduces to equation (1), the constant C having a value of 0*9. There are, however, two arithmetical errors in Laue's paper; if these are corrected C becomes 1-42. C involves another constant (v; by using a different method for evaluating 0) we have obtained a value of 1*0 for C. Details of the calculation are given in Section V.