optical axis

By rotating the lower plate each of the objectives can be brought successively in to the optical axis of the microscope.

The optical axis of a telescope, or an imaginary line passing through the centre of the tube.

The whole apparatus may be raised and lowered in the optical axis by means of a screw.

The stage can be brought in a position vertical to the optical axis and the metal hand rests laid on top of the stage.

When the spar is cut and used in a certain direction, we see but one mark, and such a position is called its optical axis.