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