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(c) How can the statements made in section 2 about focusing be reconciled with the solutions of the phase problem which we have just listed and described under (b)?

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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Next: (d) What determines the accuracy of Up: 5. Some Practical Questions Previous: (b) How can we by-pass or

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