Switch to the 10X objective. Without moving the coarse and fine focus knobs, adjust the focus with using the diopter correction ring in the side of your dominant eye.
Next, turn the diopter correction ring with the eye opposite to the dominant eye to focus.
When focusing the 10X objective lens, slowly rotate the diopter correction ring on both left and right eyepieces, and adjust to the optimal position so that there is no diopter difference between the left and right eyes even when switching from 10X to 40X.
Completing the diopter adjustments
Your adjustment is complete if you do not feel any differences between your left and right eyes when you look into the eyepieces using the 10X and 40X objectives.