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.

**Copyright © 1997 International Union of Crystallography**