【正文】 nto sharp focus.6) On a binocular microscope, the eyepieces can now be further adjusted。 one of them will focus separately, as many people have slightly different focus in each eye. The image should be brought into focus in the non adjustable eyepiece first using the fine focus knob, and then the other eyepiece adjusted so that it is also in focus.7) If a higher magnification is required the appropriate lens can be moved into place, and the image can be refocused using the fine focus knob. When using higher magnification lenses, the coarse knob should not be used for focussing, due to the risk of crashing the lens in to the specimen.8) If different areas need to be imaged, many microscopes have mechanical controls that allow repositioning of the specimen under the lens. If the specimen has not been mounted flat, making such movements at high magnification brings the risk of crashing the lens into the sample.Figure 13 – A reflected light microscopeTransmission Light MicroscopyIn transmission light microscopy the sample is illuminated and imaged from opposite sides, and will therefore only be effective on thin slices of transparent materials, which are usually mounted on a glass slide. The same protocol given above can be used, and a transmitted light microscope is shown in Figure 14.Figure 14 – A transmitted light microscopeMeasurement of FeaturesWhen showing images taken in a microscope it is essential that some indication of the magnification be given. This could be by giving the magnification as a number, . 100. When calculating the magnification it is important to remember that eyepieces often also magnify the image in addition to the magnification of the lens. It should be checked whether the light path to the camera includes similar additional magnification or not. Another way to indicate the scale of the image is to include a scale bar。 a line drawn on the image with an indication of the length it represents. The procedure for determining the size of a scale bar or measuring features directly is the same.The easiest way of measuring the size of a feature under a microscope is to relate it to the size of the field of view. The simplest way of achieving this is to measure the size of the field of view at a low magnification, and then scale the size appropriately as the magnification is increased. The field of view can be measured approximately by looking at a ruler under the lowest magnification lens. Accuracy can be improved by using a graticule (Figure 15). A graticule is a slide with a very fine grating which, if metric, will usually measure 1mm across, and is divided into 100 segments, . each segment is 10 181。m across. This allows much greater accuracy in measuring the field of view, and so greater accuracy in measuring features.Figure 15 Metric graticule in polarised lightOn some microscopes, a scale bar is superimposed on one of the eyepieces, which can be used to further improve the accuracy of measuring feature sizes. The scale bar can be calibrated by observing either a graticule or a ruler at a low magnification. For example, if 1 division is equivalent to 20 mm with a x5 magnification lens, each division is equivalent to 2 mm with a x50 magnification lens. By measuring a feature using the scale in the eyepiece, the actual size of the feature can be calculated by knowing the width of the divisions in the eyepiece. The scale bar on the eyepiece is particularly useful because it can be rotated and so both widths and lengths can be measured without rotating the specimen.Other Optical Microscopy TermsThere are numerous details to advanced optical microscopy, which are beyond the scope of this course. The list below gives brief details of some of the other terms that may be encountered, and the reader is invited to consult the reading list for more details.? Bright Field (.) illumination is the most mon illumination technique for metallographic analysis. The light collected to make up the image is that reflected directly from the specimen. This type of illumination produces a bright background for flat surfaces with the nonflat features (pores, edges, etched grain boundaries) being darker as light is reflected back at an angle.? Dark Field (.) illumination is a lesser known but powerful illumination technique. Here the unscattered beam is excluded from the image, producing a dark background for flat surfaces with the nonflat features (pores, edges, etched grain boundaries) being brighter as light is reflected at an angle back into the objective.? Differential Interference Contract (DIC) (also known as Nomarski Interference) is a very useful illumination technique for enhancing specimen features. DIC uses a Nomarski prism to separate a polarised light source into two beams which take slightly different paths through the sample. Where the length of each optical path differs, the beams interfere when they are rebined, and height differences are visible as differences in colour.? Optical Interferometry is a technique that provides precise details of a material’s surface topography. The principle uses a beam of light split in two. One beam is focused on the specimen and the second beam on an optically flat reference surface. The two reflected beams are then rebined and pass through the eyepiece together, with their interference creating contrast where the difference in optical path between the beam going to the sample and that to the reference differs by a whole or a half wavelength.Sample Preparation Route for SteelAs an example, a possible sample preparation method for a fairly typical steel will now be given. Such a process is likely to work reasonably well for most metals, although if poor results are achieved it is important to consider if another treatment would be more appropriate.SectioningAbrasive cutting with SiC bladeMountingHot mounting with (.) a phenolic resinGrindingPaperLubri