The Walton and Beckett graticule is used for counting fibrous dust (e.g. asbestos or glass fibres) and is particularly useful where the majority of fibres to be counted are shorter than 5 micron. The circle is divided into four by two diametrical lines scaled in units of 5 and 3 microns respectively. 3 and 5 microns are the critical measurements of fibre lengths used in fibre counting. Unlike the usual globes of other prticle graticules the Walton and Beckett had a series of shapes to compare objects with. These shapes have been designed for comparison with fibres, especially since they incorporate an aspect ratio of 3:1 or 5:1 essential for such analysis.
Reference: W.H.Walton and S.T.Beckett, Occupational Hygiene. Vol.20 pp 19-23/. "A Microscope Eyepiece for the Evaluation of Fibrous Dusts."
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