Until recently, nearly all crystal–structure determinations with X–rays were carried out using photographic methods. Although the majority of crystal structures are still solved using photographic data, the past few years have witnessed profound changes in experimental techniques with the introduction of single crystal diffracton ters and with the application of automation procedures to their control. These changes, together with advances in crystallographic computing techniques, portend a sharp increase in both the quantity and quality of future crystallographic work.

In this chapter we shall sketch first the development of photographic and counter methods of measuring the set of X–ray intensities diffracted by a single crystal. This is followed by a survey of single crystal techniques in neutron diffraction and by a comparison of these techniques with X–ray methods. The chapter finishes with a discussion of automatic diffractometers and of the accuracy, speed and cost of counter methods. Our main aim in this chapter is to introduce briefly many points which are explained more fully later in the book.

X–ray techniques for measuring Bragg reflexions

The first diffraction pattern from a crystal, copper sulphate, was recorded on a photographic plate (Friedrich, Knipping & von Laue, 1912). Shortly afterwards, the ionization spectrometer (Bragg & Bragg, 1913) was developed and used both for the measurement of the wavelengths of X–ray spectra and for the determination of crystal structures.