sharp focus superimposed on the microscope field of view.

In order to evaluate the asbestos fibre concentration of the
sample, the reticle must be properly calibrated. The calibration
of the reticle will vary with the specific eye-piece - objective -
reticle combination being.used. Should any of these be changed
(i.e. disassembly, replacement, zoom adjust, etc.) the combination
must be recalibrated. The stage micrometer is used to calibrate

the porton reticle:
- clip the stage micrometer onto the mechanical stage

- focus on the micrometer scale at the 40-45X objective
magnification

- align the left boundary of the large rectangle on the
reticle with the first scale division of the micrometer

- determine the length of the large rectangle in mm. by
counting the number of scale divisions of the micrometer
along this length.

Having determined the length of the large rectangle, the field
area (f.a.) and circle diameters (Dp) can then be calculated

from the following relationships:

- the length of the large rectangle is defined as 200L units
- the width of the large rectangle is 100L units ~

- thus the ieee of the counting field (f.a.) is (1001)?

- the diameters of the circles are given by the equation
Dn = uf 2" where n = circle number

e.g. suppose the length of the rectangle as measured by
the stage micrometer was .132 m,

then: 200L = .132 mm
L .00066 mm. = .66 ym

1. counting field area (f.a.)
f.a. = (100L)2 = .00436 mm2

2. circle diameters (D)

/ 1
Dd) = .66 2° = 3.93-4pim
D, = .66 {2 - 1.32 pm