. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 2. Dark-field micrograph of part of a sample of mine dust showing the distribution of sodium chloride. scopy is used. The system is illustrated in figure 1. O is the object from which a number of co-axial cones of electrons (Bragg reflections) emerge. A diaphragm (A) with a narrow annular aperture, such as that shown at the right, is introduced between the object and the objective lens. By varying the distance of the diaphragm from the object any one of the cones of electrons can be made to pass through the
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 2. Dark-field micrograph of part of a sample of mine dust showing the distribution of sodium chloride. scopy is used. The system is illustrated in figure 1. O is the object from which a number of co-axial cones of electrons (Bragg reflections) emerge. A diaphragm (A) with a narrow annular aperture, such as that shown at the right, is introduced between the object and the objective lens. By varying the distance of the diaphragm from the object any one of the cones of electrons can be made to pass through the aper- ture, all other electrons being stopped by the dia- phragm. In this way a dark-field image is obtained which is formed by only one cone of electrons cor- responding to one ring of the diffraction pattern. Only those parts of the object which contribute to the particular cone of electrons selected are visible in the image, appearing bright on a dark background. Best separation of the Bragg reflections can be obtained by means of an aperture in the back focal plane of the objective lens, for it is here that the diffraction rings are sharpest. However, it is difficult to design an annular aperture the size of which can be varied at will. Owing to the very small lens aper- tures, and the small field examined, in electron micro- scopy it is not essential to locate the aperture dia- phragm in the back focal plane. Resiihs.—To test the effectiveness of the dark- field method a sample of sodium chloride crystals ranging from 500 A to 1000 A in size was prepared on an aluminium film with a grain size of 100-200 A. Dark-field micrographs were obtained using reflec- tions first from the aluminium then from the sodium chloride. In the first only particles of 100-200 A were observed and in the second only particles of 500-1000 A. Figure 2 is a dark-field micrograph showing the distribution of sodium chloride in a sample of mine dust. A limited number of samples of mine dust have bee
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