# Experiment vs manipulation (Rindler Page-41)

As just another application of the midframe lemma, we end this section with a proof of the obvious-seeming 'reciprocity theorem' which asserts of any pair $$S$$ and $$S'$$ of inertial frames, using standard identical standards of length and time, that each ascribes the same velocity to the other. For the manipulation performed in $$S$$ to determine the velocity of $$S'$$ can be regarded as an experiment in the midframe $$S''$$. By a suitable 180- rotation in $$S''$$ this experiment is transformed into a manipulation in $$S'$$ for determining the velocity of $$S$$. And by the assumed isotropy of $$S''$$ , the two outcomes must be the same.

Why does Rindler differentiate between 'experiment' and 'manipulation' in the above?