The Theory-Dependence of the Use of Instruments in Science

The idea that the use of instruments in science is theory-dependent seems to threaten the extent to which the output of those instruments can act as an independent arbiter of theory. This issue is explored by studying an early use of the electron microscope to observe dislocations in crystals. It is shown that this usage did indeed involve the theory of the electron microscope but that, nevertheless, it was possible to argue strongly for the experimental results, the theory of dislocations being tested, and the theory of the instrument, all at the same time.