When the optical axis of the glasses is perpendicular to the main axis of the spectrometer with the aid of the plane mirror adjuster, why should the stage be rotated by 180 degrees and adjusted repeatedly? If you only adjust it once, you can only ensure that the telescope is vertical to the plane mirror, but you can't ensure that the telescope is horizontal and the plane mirror is vertical. Repeated adjustment of 180 degrees can gradually reduce the included angle between the plane mirror and the lead straight line until the mirror reaches the vertical state, thus ensuring that the telescope is in the horizontal state.