"显微镜的计数线" can be translated into English as "counting lines on a microscope" or, more specifically in a scientific or technical context, "micrometer grid lines (on a microscope slide/stage micrometer)" (if referring to the calibrated lines on a micrometer scale used for measurement).
Counting lines on a microscope: This is a straightforward translation, emphasizing the lines used for counting or measurement under a microscope.
Micrometer grid lines: This is more precise when referring to the calibrated lines on a micrometer scale (often on a microscope slide or stage micrometer) used for accurate measurements.
Choose the translation that best fits the context in which you are using the term.