An apparatus is disclosed for use in a centrifuge apparatus to rotate a fluid tube about its longitudinal axis while the rotor of the centrifuge apparatus is rotating the fluid tube in a rotational direction transverse to the longitudinal axis of the tube. The apparatus comprises a cam which is mechanically coupled to the fluid tube, and a cam follower configured to apply a driving force to the cam to cause the cam to rotate, which in turn rotates the fluid tube about its longitudinal axis. The cam follower applies the driving force to the cam in response to changes in the magnitude of centrifugal force applied to the cam follower caused by increases and decreases in the speed of rotation of the rotor. The centrifuge apparatus can therefore obtain optical readings of the centrifuged sample from different locations about the circumference of the fluid tube.