This can be done by attaching the laser to the arm used for rotation and using it to find the extended axis of rotation on a suitable surface, in a similar way to that which was used for the vertical spindle. A string is then tightly stretched between this point and the centre of the spindle used for rotating the arm. This should be horizontal - one way to check this is with a water level (basically water in a transparent flexible tube). In addition it should pass through the polar axis of the goniophotometer. If it does not then the necessary adjustments have to be made.