= 9 = with the shape of the filter wedge. Using the tweezers, grasp the sector to be mounted by its unexposed outer edge and place the sector, exposed side up, on the mounting medium. Carefully place a clean coverslip over the ‘filter sector. Do not reposition the cover slip once contact has been made. Label the slide with a sample number and mounting date. The sample should become transparent within about 15 minutes, but approximately one hour or longer should be allowed for all graininess to disappear before the slide is counted. The slide should be counted within three days. 5. (i) (ii) OPTICAL SYSTEM -— ADJUSTMENT AND CALIBRATION Microscope Adjustment: The microscope is adjusted, according to manufacturer's instructions, to produce positive phase contract images using Koehler illumination. The following general guidelines must be observed: = the light source must be in focus and centered on the condensor iris or annular diaphragm ° = the object for examination must be in focus - the illuminator field iris must be in focus, centered on the sample, and opened only to the point where the field of view is illuminated - the phase rings (annular diaphragm and phase-shifting elements) must be concentric Porton Reticle (Graticule) Guest tone -- (see page 29, figure II for Cassiar graticule) The porton reticle is a glass plate inscribed with a series of eircles and rectangles (page 28). This reticle is used to determine the fibre lengths and also to define a field area in which the number of asbestos fibres can be counted... The fibre lengths are determined by comparison with the circles on the reticle. The square on the left of the reticle, which is divided into six rectangles, is the counting field area (f.a.) in which fibres are counted. The porton reticle rests on the field limiting diaphragm in the Huygenian eye-piece, and remains in