In transmission electron microscopy (TEM), it is often desirable to focus our attention on certain cells or structures that are sparsely distributed throughout a large tissue block. One method that is often used is to serially section through a large tissue block and then scan each thin section on the TEM for the cell or structure of interest. A much simpler and far less tedious method is to cut thick sections of the resin block, screen the sections using light microscopy for the cells or structures of interest and then somehow reembed only those sections with the cell or structure of interest so that they can be thin sectioned for TEM.

Although the secondary electron (SE) signal is still the most commonly used signal in scanning electron microscopes (SEMs), the backscattered electron (BSE) signal is now in wide use. Imaging both atomic number and surface topography have been the major applications of BSE detectors, with some applications in channelling, magnetic contrast and similar specialized applications. Over the last few years, low voltage BSE imaging has been used for imaging surface features to a depth of a few nm. But the BSE signal contains much more information and new techniques are being developed to take advantage of its versatility.

During the early 1970's there was often heated debate over the quality of analytical performance attainable with Energy Dispersive (ED) systems using solid state detectors. Many scientists, particularly the more practical ones, were convinced that only WDX was reliable; while those with perhaps more imagination were convinced EDX was the future in microanaiysis. At that time, both groups were right.

EDX has seen remarkable improvement since those early days. At that time ED spectrometers were limited to analysis of elements above atomic number 10, and state-of-the-art resolution was improving to about 155 eV at Mn. Overlaps were handled strictly in quantitative analysis routines and even then only by use of simple overlap factors. Wavelength Dispersive (WD) spectrometers, although slow and difficult to operate, were required for light element analysis, to perform analyses when overlaps were present or suspected, and to detect trace elements.

Success in photomicrography depends, not only upon the acquisition of a technique, but also upon the selection of equipment. Unless the equipment is provided with certain facilities and controls, it is impossible to create optimum final image quality in the object plane.

We shall direct our attention to that part of the equipment which - next to the performance of objective and eyepiece - has the greatest influence upon the quality of the image, viz., the illumination system.