Search

In principle, a finite-element calculation of a magnetic or electrostatic structure has now achieved an accuracy that is difficult to check experimentally. However, incidental errors may occur in practice due to shortcomings in the computer itself, in the program or in the initial setting up of the data. Some common errors, culled from the literature, are reviewed; ways of recognising their presence are suggested. Some test structures and computing procedures are also put forward for estimating the accuracy of a particular calculation. A software “BH Tester ” has been devised that is intended to help the operator to check default B  H curves that may be stored in the computer. This can also be used to exercise a given program at its limits and hence reveal any weaknesses. Finally, a series of tests has been carried out to assess the relative merits, in speed and accuracy, of the Gaussian elimination method of solving the matrix equations and that of the imcomplete Cholesky conjugate gradient (ICCG) method.

The finite element method of calculating the magnetic fields of electron lenses is critically reviewed in the light of recent developments in practical lens structures. Some systematic errors have been found in current computing techniques. A revised computational procedure has been developed which detects such errors, and provides means for their correction.

SUMMARY Liquid metal ion sources (LMIS), in view of their very high brightness, have opened new horizons in the field of ion beam technology and microscopy. In this paper some basic ion-optical problems of LMIS-based systems are discussed, and also some of the fundamental mechanisms concerning the operation of LMIS. While a type of field-evaporation is a strong candidate for the main ion emission mechanism, a space-charge based theoretical treatment successfully predicts the current-voltage curves of LMIS of low liquid flow impedance. These conclusions are based on the most detailed measurements to date of the ‘peak energy deficit' of the ion energy distribution, and on the comparison with theory of the current-voltage curves of a variety of LMIS. Finally, from data on gallium LMIS it is shown that an optimum operating current exists around 2 μA.

The development of the electron microscope in the advanced form we know today is the result of a loosely organized but effective world-wide scientific and technological collaboration, that readily transcends national and cultural barriers. The birth of the subject was undoubtedly in Berlin but the United Kingdom has played a prominent part, sometimes in an accidental way, at critical periods in this development both at the scientific and manufacturing level. In 1927, for example, G.P. Thompson established experimentally the wave nature of the electron and its scattering and diffraction by thin foils. This followed his invention of the electron diffraction camera at a time when De Broglie's postulates on the wave of nature of the electron were not enjoying much support, even in his native France. C.W. Oatley in Cambridge University was successful in introducing the first commercial scanning electron microscope (SEM) where several commercial firms had previously failed. D. Gabor invented electron holography in a moment of boredom while watching a tennis match. The U.K. has also played a key role in fostering international collaboration by means of conferences and exchange visits between electron microscopists. The present paper concentrates on some of the critical developments that took place in the U.K. in the commercial development of transmission electron microscopy, in scanning electron microscopy and more recently in high resolution electron microscopy. These developments are placed as far as possible in a world-wide context.

The electron microscope surpassed the optical microscope in resolving power 50 years ago, though at the time it did not receive an enthusiastic reception from scientists, technologists or members of the medical profession. Today atomic resolution has been achieved and high resolution electron microscopes are making an impact in materials science, in materials technology and in diagnostic medicine in the same way that optical microscopes did in the 19th century.

SUMMARY The unpredictable development of the electron microscope is reviewed in the context of the 150th Anniversary of the Royal Microscopical Society. Abbe convinced himself that an electron microscope could never be constructed. Later, J. J. Thomson established the minute corpuscular nature of the electron and Rutherford realized that such beams can reveal the nature of the atom; his student H. G. Moseley invented electron probe analysis, while looking for X-ray spectra, but the technique did not seem very practical. G. P. Thomson invented the electron diffraction camera, but it was of limited use without an electron lens. In 1931, Knoll and Ruska built a two-stage TEM with magnetic lenses and understood where Abbe went wrong. Although Ernst Ruska surpassed the resolving power of the light microscope in 1933, the first commercial TEM was manufactured in the U.K., leading to many further commercial developments. The realization of a digital computer with a stored program, now indispensible in microscopy, was also achieved first in the U.K. Gabor's electron beam holography, invented in Rugby in 1949, was not a challenge to the conventional TEM at first, but recently atomic resolution holography has been achieved in Abbe's native country, using a digital computer to process the hologram, ironically making use of the very principles laid down by Abbe in the last century for the light microscope.

SUMMARY Methods of analysing electron micrographs are reviewed, with special reference to the needs of the practising electron microscopist. It is concluded that a simplified optical diffractometer and a projection ‘Periodograph’ can be particularly helpful, especially in biological investigations, to which extensive reference is made in the bibliography.

Manfred von Ardenne is remembered principally as the builder of the first scanning (transmission) electron microscope. This was only one of his many contributions to electron microscopy design, which include stereo imaging, high-voltage electron microscopy (with 1 MV envisaged) and microtomy.

This year's Nobel prize concentrates on experimental physics at the limit of human ingenuity, followed up by the realisation of new and unexpected research tools – the transmission and scanning tunnelling electron microscopes. Speculation about such instruments goes back to 1878 when Professor Ernst Abbe, musing on his new, definitive theory of the optical microscope, made the following pronouncement: 'Scanning today's scientific horizon, there seems no chance of designing any kind of microscope with a resolving power better than that of an optical microscope, which is now limited by the laws of diffraction and cannot therefore be improved further by human skill.

Laurie Taylor 1986 Stoke on Trent: Trentham Books 149 pp price £4.95 (paperback) ISBN 0 948080 08 6 For the last eight years Laurie Taylor has been writing an off-beat column in the Times Higher Education Supplement, charting in brilliant flights of fancy the dismantling and restructuring of the traditional British university system. For those who have experienced the confusion and absurdities of the period, this volume, a hilarious distillation of his THES contributions, is compulsive reading.