"显微镜的计数线" can be translated as "counting lines on a microscope" or more specifically as "micrometer lines (on a microscope stage or reticle)".
Counting lines on a microscope is a straightforward and commonly understood expression.
Micrometer lines refers to the precise, calibrated lines used for measurement or counting under a microscope, often found on the stage micrometer or in the eyepiece reticle.
Choose the term based on the context and level of specificity required.