An actuation device driven by the adjusting motion of a first carriage is provided and, because of the adjusting motion of the first carriage from the first position into the second position, the adjusting motion of a second carriage can be driven from the measurement point to the rest position, and because of the adjusting motion of the first carriage from the second position into the first position, the adjusting motion of the second carriage can be driven from the rest position to the measurement point.