Reading microscope is a measuring tool invented by people (mainly used to measure micro-scale). As a tool, it should meet this condition: make people's work as convenient as possible. It is based on this condition that when setting the reading microscope scale, people have consciously made the measured length the length of the observed object. More specifically, the scale width we see in the objective lens is the actual magnification width of one millimeter. Use this magnified ruler to measure the measured object with the same magnification, and the measured length is the actual length of the object.