Since the first century A.D., mankind has found ways to see better1. During the next twelve centuries, convex lenses were made from clear minerals for eyeglasses, in order to overcome far-sightedness. Then, in the 1300's, clear, artificial glass became available for the same purposs. By the sixteenth century, concave lenses were made for the near-sighted. It was not until the 17th century that a combination of lenses led to the sciences of astronomy and microscopy.

One of the earliest important microscopists was the Englishman Robert Hooke (1635-1703). He owned a compound microscope with an objective lens and an eyepiece, much like Galileo's telescope.

Kent Christensen documents the values of glass photographic plates1 and also their demise. As a former user of glass plates for microscopy, radiography, autoradiographyandspectroscopy, I sympathize with Kent. Today there are substitute sheet and or roll films for most of the glass plate applications. For example, [ have found Kodak's "Trnax" films as sensitive as lantern plates for soft X-rays and nearly as good for resolution. Tmax is also a good general purpose emulsion for the laboratory, with the possibility of reversal development for slides and availability in a range of formats including 120 and four by five. Perhaps we should recognize the passing of the glass plate as part of current times and inscribe "RIP" over our hallowed collection of glass negatives. After all, many predict that electronic photography will eventually replace all forms of chemical photography.

A number of important imaging applications, including neurobiology and precision motion analysis, require a digital camera that provides both high performance (low noise, extended linear gray scale) and high frame rates. In recent years, manufacturers have responded to this need with faster digitization/readout electronics - as high as several MHz (Mpixels/second). When combined with on-chip binning, this has enabled frame rates up to 30 frames/second from some high performance cameras. Unfortunately, even this is not fast enough for situations where two images must be acquired virtually simultaneously, or where a rapid dynamic event needs to be captured with a fast burst of images.

LaB6 and CeB6 cathodes are widely used as high-brightness, highstability, long-lifetime cathodes in a variety of electron beam appiications. Two of these factors - stability and lifetime - are critically dependent on proper cathode-mounting design.

When the benefits of LaB6 as an emitter material were first investigated, the LaB6 was indirectly heated by an electron bombardment coil. This design was cumbersome and did not perform well. Wire mounts, such as those used for tungsten emitters, were developed (and are stili used today); however, they have many drawbacks. Their stability and lifetime are limited because the thin wire mount moves easily and will eventually break under the changing temperature conditions encountered during operation.

Each year we show hundreds of students and visitors magnified images of common objects in our SEM. the best samples to use in SEM demonstrations for children are those which have detail that can just barely be seen with the naked eye or with a light microscope. Ideally, the sample would be something with which the child is already familiar, and one which can be explored at several different magnifications.

The simplest specimen is a penny attached to a stub by carbon or silver dag. Mount the coin such that the back side, showing the Lincoln Memorial, faces upward As shown in Figure 1, the statue of Lincoln inside the memorial can be seen with the naked eye.

Part 1 of this article series (October 1994 issue) covered the performance features of various tip shapes and explained why the <100> crystal orientation is the industry standard. Part 2 (January/February 1995 issue) covered cathode mounting design and explained how the Mini Vogel Mount cathode design provides high-stability and long-lifetime performance.

LaB6 and CeB6 cathodes are used as high-brightness, high-stability, longlifetime cathodes in a variety of electron beam applications. A high-brightness source provides small spot sizes for high resolution and improved analytical results from high beam currents.

Over the last several years the long-awaited revolution in direct-digital readout systems has begun, with the introduction of efficient slow-scan CCD cameras. Earlier, the introduction of video cameras to electron microscopes had brought a quantum leap in the speed and efficiency of carrying out a host of operations. The high sensitivity of the video cameras provided the ability to see the image in much more detail and at a lower beam intensity than had been previously possible by viewing the fluorescent screen. The ability to assess, on line, characteristics such as specimen quality and image focus, even qualitatively, gave feedback to the operator that previously took hours to obtain. Due to the low resolution of these video systems, however, they were rarely useful for data recording.

Let's face it, optics is a fascinating field, but it can bore you to death. My purpose in writing these articles is to show that there are simple, intuitive ways to look at microscopes that can illuminate rather than bore. I have been an optical engineer for many years and have observed many unique ways of looking at optical instruments. I hope that I can share my enthusiasm and insight for light and its manipulation and give some insights that microscopisis may have missed. The microscope is particularly interesting because the glass lenses often do more than one job at the same time. Most of the principles of optical microscopy are directly transferable to electron microscopy, and I will point out specific cases as I go along.

Ion millers are a pain in the neck! They are designed to remove material from the sample. This means: a) that they always remove material from other places too and b) that the material gets deposited somewhere else. The result of this is that ion millers are inevitably high-maintenance instruments.

Therefore it is important to make the most of them when they are in operation. To aid in this quest, two things can be helpful: to get the beam well aligned and to know the milling rate.

An ion gun, however powerful the beam it produces, is of no use if the beam misses the sample. A means of checking the alignment is therefore of great importance.

As laboratories like the Materials Analysis User Center (MAUC) in the High Temperature Materials Laboratory, at Oak Ridge National Laboratory move from analog to digital imaging systems, the process for acquiring, processing, analyzing, and publishing research results is changing. In this case since original scientific data exist only in digital form, analog systems for gathering, staring, and transmitting data are being set aside in favor of digital systems. In order to adequately protect original scientific data and to ensure that digital laboratories have the same (or greater) functionality as analog laboratories, scientists must focus on building information systems that make data highly available, highly reliable, and quickly accessible. Meeting these three objectives imposes special requirements for both data storage systems and data networks.

Last months issue of this publication contained the second article article in this series on digital imaging far TEM - and listed factors that may be stimulating the recent interest in digilal imaging for TEM. Most of the factors are concerned with issues of functionality. A digital imaging system can enhance TEM operation, image storage, image processing and communication. Significantly, the surge in interest was not linked to claims that digital images have better detail than images captured using sheet film. This is a pivotal issue for some microscopists. Nevertheless, my experience has been that many rnicroscopists, even skeptical microscopists, are surprised at the quality of output from a modern digital imaging system when using a high quality output device to produce hard copy images.

Most workers wishing to prepare material for microscopy will study a limited range of organisms, and already be familiar with raising, culturing or collecting the species in question because of their research interests or adopted field of study. For those new to microscopy who have not yet defined a field of interest, it is suggested that they read a practical introductory text such as that of Grave (1991).

The diversify and abundance of animal, plant and microbial life available for collection means that gathering material can be a relatively simple task. Nevertheless, a methodical approach ensures that specimens are less likely to suffer damage and full details of their natural habitat are known, which will place any serious study into scientific context.

Teaching a large number of students simultaneously to use a scanning electron microscope (SEM) has traditionally been difficult. A typical classroom scenario would be of one student seated in front of the SEM next to the instructor while the remaining students stand idly behind the operating student. The truth of the oft repeated sentiment expressed by microscopists, "There is nothing more boring than watching someone else do microscopy", would be proven time and time again in such training sessions as the uninvolved students would quickly lose interest and either fall asleep or talk among themselves.

The ability to view thick material using accelerating voltages higher than 100 KeV has greatly expanded the usefulness of electron microscopy. At or above 300 KeV, intact cultured cells and tissue sections as thick as 5µm can be visualized. However, under normal viewing conditions this increased depth produces confusing superposition of structures located at different planes along the Z axis. In order to separate spatial information in different planes, specialized procedures such as stereo pairs or animated tilt series have been employed. Although rapid, these procedures do not allow quantitation of parameters such as size, volume, or circumference. Computed Tomography produces a computer generated 3-D reconstruction from which quantitative data can be easily extracted.

X-ray spectrometers give the capability to determine chemical element composition in electron microscopes. The semiconductor with the most experience as an x-ray detector is silicon. Silicon is the most highly developed material on earth, and has a lot of good things going for it, but for some applications we crave something with other good properties. For example, for room temperature detectors it would be best to have a semiconductor with a wider band gap. For higher resolution it would be better to have a semiconductor with a smaller band gap. For these reasons a number of other semiconductors have been developed as x-ray detectors. In this article I will talk about narrow band gap semiconductors. Next time I will discuss large band gap semiconductors.

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.