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5. The Resolution of the Structure

If some of the radiation scattered by an object under examination with a microscope escapes rather than being recombined to form an image, the image so formed will be an imperfect representation of the scattering object. Fine detail will remain unresolved. Similarly, with X-rays, if the diffraction pattern for the customary wavelengths is observed only out to a relatively small scattering angle, the resolution of the corresponding image reconstructed will be low. Furthermore, the resolution will be limited by the wavelength chosen even if the entire pattern is observed. The `resolution' obtained is usually expressed in terms of the interplanar spacings $d = \lambda / 2 \sin \theta$ ,corresponding to the maximum observed $2\theta$ values. The effect of changing resolution on the appearance of an electron density map is shown in Fig. 5. Often, with macromolecules, the order does not persist from unit cell to unit cell to high resolution. This lack of high resolution may also be observed when data are collected for crystals near their melting points.

**Figure 5**
$\begin{figure} \includegraphics {fig5.ps} \end{figure}$

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