The four techniques agree very well with the statements about focusing made in section 2. In (i) we clearly have to know a good deal about the object in order to focus. In (ii) (as was pointed out in the discussions of 5(a) above) we can only interpret a Patterson map if we know something about the object even if-- in the simplest possible case--our only knowledge is that the object consists of discrete atoms rather than of a continuous distribution of electron density. In (iii) our heavy atom is analogous with the hair or speck of dust that we know is there: if we focus on it we can assume that the rest will be in focus. In (iv) the mathematical relationships used are all found to depend on specific assumptions about the object--usually that the scattering is everywhere real and positive and that discrete spherical atoms make up the object.

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