= 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