1. Open the cover and carefully wipe the detection prism with a soft cloth 2. Drop distilled water on the prism, turn the zero adjustment screw to adjust the dividing line to 0% of the scale, and then wipe it dry 3. Drop the solution to be tested on the prism and close the cover to avoid bubbles and spread the solution on the surface of the prism 4. Point the light entrance plate of the instrument at the light source or bright place, observe the field of view through the eyepiece, turn the eyepiece adjustment handwheel 5. Make the blue and white dividing line of the field of view clear, and the scale value of the dividing line is the measured value of the solution