  During a continuous measurement, quantum systems can be described by a
stochastic Schr\"odinger equation which, in the appropriate limit, reproduces
the von Neumann wave-function collapse. The average behavior on the ensemble of
all measurement results is described by a master equation obtained from a
general model of measurement apparatus consisting of an infinite set of degrees
of freedom linearly interacting with the measured system and in contact with a
reservoir at high temperature.
