At the passage through the two slits of the second screen S 2, the wave aspects of the process become essential. In fact, it is precisely the interference between the two terms of the quantum superposition corresponding to states in which the particle is localized in one of the two slits which produces zones of constructive and destructive interference (in which the wave function is nullified). It is also important to note that any experiment designed to evidence the " corpuscular " aspects of the process at the passage of the screen S 2 (which, in this case, reduces to the determination of which slit the particle has passed through) inevitably destroys the wave aspects, implies the disappearance of the interference figure and the emergence of two concentrated spots of diffraction which confirm our knowledge of the trajectory followed by the particle.