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1. Introduction

In my view the basic problem in presenting X-ray diffraction to non-specialist audiences is to remove some of the atmosphere of mathematical difficulty and mysticism and to show, first of all, that the processes involved are, in principle, identical with those of microscopy.

My suggested approach to this problem is to use optical analogues at quite an early stage: the mathematics can be filled in easily enough once the essential ideas have been grasped--or of course it may be that a non-specialist group will not need the mathematics anyway. I shall illustrate this pamphlet by referring to illustrations in the Atlas of Optical Transforms which was published in 1975 by Bell for the Unesco pilot project of the Teaching Commission, which really initiated the idea of these pamphlets.

I usually begin by drawing attention to the basic steps that occur in all processes of image formation: the first is scattering of the radiation and the second is recombination of the scattered beams. The basic idea can be illustrated with an ordinary 2" $\times$ 2" slide projector. If the lens is removed so that a diffuse patch of light is seen on the screen even though a slide is in place, it will be clear to an audience that all the information that is contained in the slide must be available in the patch of light on the screen although it is not readily decipherable. Clearly the lens cannot `know' anything about the slide and yet as soon as it is placed in the correct position the nature and detail of the slide are revealed. All that the lens can do is to rearrange the information so that it is immediately understandable to the eye and brain.

Copyright © 1997 International Union of Crystallography