354 



J. H. TALBOT 



Fig. I. Ray diagram illustrating the principle of the dark- 

 field system. At the right is shown the special annular aper- 

 ture. 



are collected with a thermal precipitator directly on 

 beryllium films about 30 A thick. After collection 

 they are heated to 550 C to remove organic matter 

 and condensed oil vapour. At the same time the 

 beryllium oxidizes to form a crystalline oxide film, 

 the diffraction pattern of which is used to calibrate 

 the diffraction pattern of the mine dust. 



Using a three-stage electron microscope, bright- 

 field micrographs and the diffraction patterns of 

 several parts of the specimen are recorded. The 

 constituents are identified by comparison of the 

 diffraction patterns with standard diffraction pat- 

 terns of substances, the presence of which is sus- 

 pected, or by comparison with the A.S.T.M. index. 



Having determined the various chemical constitu- 

 ents present in a sample by electron diffraction, it 

 may be desirable to establish in which parts of the 

 specimen the various constituents are present. For 

 example, assume that several constituents have been 

 identified in a sample containing a large number of 

 particle sizes and shapes. It might be that one or 

 other constituent is confined to particles of a partic- 

 ular size range or of a particular shape. If this is so, 

 and the relation between each constituent and the 

 particle size range or shape can be established, then 

 it is a simple matter to determine the number and 

 the sizes of the particles of each chemical compound 

 present. 



To establish the relationship, if any exists, between 

 a particular constituent and any peculiarities of the 

 sample, a new system of dark-field electron micro- 



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 

 been examined. Preliminary indications are that the 

 bulk of the submicroscopic particles consists of non- 

 siliceous particles. 



