There are scores of microscopes that detect different properties of a specimen. Typically we image ‘visible” properties, but, for example, even the commonly-used atomic force microscope detects physical interactions rather than “visible” characteristics, Mostafa Fatemi and James Greenleaf have introduced the principle of imaging the elastic features of a specimen. This is done with sound waves, but we are not talking about just another acoustic microscope.

The method demonstrated by Fatemi and Greenleaf uses radiation force to image the acoustic response of a specimen to mechanical excitation. The mechanical excitation results from focusing two coaxial and confocal ultrasound beams of slightly different frequencies onto a selected region of the specimen.

The optical examination of a rock sample in thin section is the quickest and most economical method for classifying rock type and determining which analytical route to follow.

Thin sections for transmitted light are the most common, but there are also:

Polished Thin Sections • Polished sections are used for classification and identification of minerals that cannot be determined in standard thin sections. They are also essential for microprobe analysis. Minute mineral grains are analyzed by bombarding them with a focused bean of electrons, which generate x-rays, characteristic of the elements within the grains. X-rays are identified and quantified to determine the chemical composition of minerals.

Approximately 75 % of the primary electrons will be scattered when you use a working distance of 12 mm and a pressure of 3.5 torr, and a significant fraction of these scattered electrons will hit the sample farther away from the beam target than 50 pm. (The scattered intensity is approximately given by Is/ lo = 1 – exp(-ρσL/kT) where p is the pressure, σ the total scattering cross section for electron scattering on the gas used, L the distance between the last pressure limiting aperture and the sample, k the Boltzmann constant and T the absolute temperature).

Last month I addressed the phenomena of beam scatter in the environmental and low vacuum SEM. In that article I covered how beam scatter is affected by chamber pressure, working distance, and accelerating voltage. To briefly summarize, beam scatter becomes worse at higher chamber pressure, longer working distance, and lower accelerating voltages. As the beam scatters, electrons strike the sample some distance away from the primary beam and as a result, generate X-rays from unwanted areas of the sample. It is advantageous for the analyst to keep beam scatter to a minimum to reduce the generation of these X-ray signals. Under conditions of high chamber pressure, long working distance, and low accelerating voltage, it is possible for electrons from the beam to strike the sample on the order of a millimeter or more from the primary beam.

Complete casts of the vasculature of organs and tissues are obtained by infusing low viscosity resins into the vasculature and allowing the resin to polymerize. Dissolving away the surrounding tissue with alkali leaves a model of the intricate, three-dimensional distribution of vessels in that tissue, which is not easily obtainable by any other means, and which can then be studied with scanning electron microscopy (SEM), Because well prepared casts appear to faithfully replicate the true vascular anatomy of organs including the dimensions of vessels and details of imprints of the endothelial cells lining their lumens, they must also contain quantitative information about that vasculature.

Wood cells have strong autofluorescence in a wide wavelength band due to lignin in the cell walls. The detection of microorganisms in wood is very difficult when using fluorescent microscopy because of interferences. We have recently developed fluorescent staining techniques to differentiate fungal hyphae from wood cell walls (Singh, et al., 1997; Xiao, et al., 1997). This study was aimed at developing fluorescent techniques to visualize bacteria in wood using confocal laser scanning microscopy (CLSM). Nitrobenzoxadiazole glycerophosphoethanolamine (NBD-PE, Molecular Probes), a widely used membrane probe which accords strong fluorescence upon lipids, was compared with glutaraldehyde which had proved useful in our initial attempts to visualize fungal hyphae in wood because of the cell auto fluorescence it causes (Singh et al., 1997).