Linear eyepiece graticules

These are linear scales, typically 10 mm, divided into 100 divisions of 100 pm, or 2 mm divided into 100 divisions of 20 pm each. They are placed in the focus of the microscope eyepiece so that they are coincident with the image of the microscope slide on the microscope stage. Calibration is effected using a stage graticule, 10 mm (100 x 100 pm), 

Kohler illumination should be used [11] to give uniform illumination of the viewing plane. Using an oil immersion objective, it is possible to resolve down to about 1 pm, although a 15% oversizing is to be expected at this level due to diffraction effects. 

Ocular graticules having a linear scale are satisfactory for the measurement of linear dimensions of particles. Particle sizes obtained with a linear eyepiece graticule are best classified arithmetically hence it is most suited to particles having a narrow size range. 

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