A binocular microscope shall be used for counting and shall have the following features:
- Koehler illumination,
- its substage assembly shall incorporate an Abbe or achromatic phase-contrast condenser in a centring focusing mount. The phase-contrast centring adjustment shall be independent of the condenser centring mechanism,
- a 40 times bar-focal positive phase-contrast achromatic objective with a numerical aperture of 0,65 to 0,70 and phase ring absorption within the range 65 to 85 %,
- 12,5 times compensating eyepieces; at least one eyepiece must permit the insertion of a graticule and be of the focusing type,
- a Walton-Beckett circular eyepiece graticule with an apparent diameter in the object plane of 100 micrometres ± 2 micrometres, when using the specified objective and eyepiece, checked against a stage micrometer.
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