X rays and crystal structure . Fig. 14. Suppose now that Fig. 14 represents the spacelattice actually underlying a crystal that we aremeasuring. It is clear that the orientation of anyface which the crystal may possess is determined bythe lattice alone and not by the pattern unit which CRYSTAL STRUCTURE 53 is grouped round each point. The faces of thecrystal represent possible ways of arranging pointsof the space lattice in planes (compare p. 12, Fig. 2).For instance, in Fig. 15, we have a series of pointslying in the plane ABC, the parallelepipeds beingpictured as bricks packed so as to show
X rays and crystal structure . Fig. 14. Suppose now that Fig. 14 represents the spacelattice actually underlying a crystal that we aremeasuring. It is clear that the orientation of anyface which the crystal may possess is determined bythe lattice alone and not by the pattern unit which CRYSTAL STRUCTURE 53 is grouped round each point. The faces of thecrystal represent possible ways of arranging pointsof the space lattice in planes (compare p. 12, Fig. 2).For instance, in Fig. 15, we have a series of pointslying in the plane ABC, the parallelepipeds beingpictured as bricks packed so as to show this plane,which is known as a net plane of the Fig. 15. The same points can be arranged in any numberof ways on parallelepipeds, those in the figure repre-senting only one possible way of carrying out thearrangement. Let us, however, choose some parti-cular way of drawing the parallelepiped. We maythen call the three directions to which the edges ofthe parallelepiped are parallel, the axes of the crystal,and all its faces can be named with reference to theseaxes. The way in which the planes are referred tothe axes is as follows. In Fig. 15 we have drawn atypical plane ABC on which a set of particles lie. Let 54 CRYSTAL STRUCTURE O be the origin at a parallelepiped corner, and OA,OB, OC parallel to the axes of the crystal. Theplane ABC cuts off the intercepts OA, OB, OC fromthe axes, and if we know the ratio of these inter-cepts, we can thereby define the orientation of theplane ABC (It will be evident from Fig. 15 that netplanes of all types can be drawn by choosing for OA,OB, OC any multiples of the parallelepiped edges.)If we ca
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