When it is fitted in the "ON" position it makes the light rays converge onto the thin section placed on the stage of the microscope. In this situation one speaks of convergent light or, more normally, conoscopic illumination.

By contrast, when the convergent light lens is in the "OFF" position, the light rays no longer incide convergently but rather folow an approximately parallel path and all of them incide perpendicularly on the slide. In this situation one speaks of parallel light or, to be more correct, orthoscopic illumination.