  The measurement of a quantum system becomes itself a quantum-mechanical
process once the apparatus is internalized. That shift of perspective may
result in different physical predictions for a variety of reasons. We present a
model describing both system and apparatus and consisting of a harmonic
oscillator coupled to a field. The equation of motion is a quantum stochastic
differential equation. By solving it we establish the conditions ensuring that
the two perspectives are compatible, in that the apparatus indeed measures the
observable it is ideally supposed to.
