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(d) What determines the accuracy of reproduction of the image?

Just as in optical microscopy the limits of accuracy are set fundamentally by the wavelength of the X-rays used and experimentally by the `aperture' of the system. In the X-ray case the `aperture' means the number of terms included in the Fourier synthesis. Plate 32 of the Atlas illustrates this with an optical analogue. 32.1 on the right is an object consisting of a small square cristallite of molecules of bishydroxy-duryl methane. (32.1) on the left is its diffraction pattern. If we used the whole of 32.1 on the left we could, in theory, produce--assuming that we knew both amplitudes and phases--an exact replica of 32.1 on the right. If we restrict the terms included in our calculation to those shown in 32.2 on the left the resulting deterioration of the image is shown in 32.2 on the right. The remainder of the plate pursues this teme in various ways and it is significant to note that in 32.6 we come back full-circle to the point that, if we only include one order of diffraction on either side of the centre (32.6 on the left) the result (32.6 on the right) is one set of sinusoidally varying fringes.


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