The desirability and requirements for a specimen capable of testing the resolving power and other image characteristics of an electron microscope are discussed in detail. In this discussion, the underlying diffraction phenomena are particularly utilized. A partially graphitized carbon black is shown to satisfy the requirements extremely well and constitutes an easily prepared specimen for conducting tests of image quality in the molecular size range. The structure of the test object is known in detail with the result that readily interpretable phase contrast images are obtained. Micrographs illustrating the use of the 3.4/~ (002) spacing for magnification calibration, astigmatism and asymmetry check as well as resolving power are exhibited. The second order c-spacing of 1.7 ,~ is occasionally found in an image. The micrographs shown herein were taken with two different electron microscopes by different operators obtaining the same structural detail in the images. It is concluded that the carbon black test object offers the best possibilities for evaluating image performance of any specimen yet suggested. Introductory discussionThe rapidly increasing interest in electron microscopy of structures in the molecular size range poses several problems among which specimen preparation, contrast interpretation and performance testing of instruments are paramount. This paper is concerned with the last mentioned subject since the primary requirement is presently a knowledge of the performance and ability of the microscope itself.Although discussions and theoretical treatments of resolution and resolving power have continued for many years, there is still no general agreement on a test object for quickly evaluating image quality. Lattice images exhibiting interplanar spacings down to about 1.2 A~ have been obtained by means of optimum tilting of the illumination to minimize spherical and chromatic aberration. The resolving power so obtained is not isotropic, however, and there has been a tendency to de-emphasize these results as not indicative of the highly desirable 'point-to-point' resolution. The criterion of point-to-point or isotropic resolving power requires well aligned, axial illumination along with compensation of objective lens asymmetry and astigmatism. Yada & Hibi (1966) have demonstrated lattice image spacings of 1.81 A, using axial illumination. This is point-to-point resolving power even though the contrast mechanism be phase rather than diffraction contrast. Major factors in phase contrast are source stability and coherence, the latter being greatly improved by the use of pointed filaments as earlier shown by Hibi (1962). Since the contrast to be realized in the molecular domain of structure is predominantly phase contrast (Thon, 1966), the coherence of the source is highly important. Two methods of estimating resolving power that have been in use for some time are the edge contour width suggested by Haine (1961) and the finely divided evaporated metal specimens. The experience of a number of workers seems...

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Bethe's dynamical theory of electron diffraction in crystals is developed using the approximation of nearly free electrons and Brillouin zones.The use of Brillouin zones in describing electron diffraction phenomena proves to be illuminating since the energy discontinuity at a zone boundary is a fundamental quantity determining the existence of a Bragg reflection. The perturbation of the energy levels at a corner of a Brillouin zone is briefly discussed and the manner in which forbidden reflections may flrise at a corner pointed out. It is concluded that the kinematic theory is inadequate for interpreting electron images of crystalline films.An electrolytic method for preparing thin metal sections for electron microscopy and diffraction is introduced and its application to the structure of cold-worked aluminum and an aluminum-

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