For microscopes the resoluting magnification can be calculated by multiplying the focal lenghts of the objective lens and the eyepiece. Thus, a 4x objective lens and a 10x eyepiece gives 40x magnification. With the same objective lenses, but 20x eyepiece the magnification rises to 80x.
All of the eyepieces listed are for the standard 23.2mm focusers (except for the 30mm Zeiss standard), which is compatible with most of the biological microscopes.
The 10x eyepiece with measuring reticle can be used to calibrate the microscope and perform measurements.