The canonical momentum in the direction of the electrical current or an other ordered motion is a constant of motion in the case of mirror type perturbations of plasma configurations in which the current and the magnetic field are perpendicular to each other and in which all the macroscopic quantities depend only on one coordinate in a rectangular system of coordinates, of which the metrical tensor depends also on that coordinate only. This fact enables one, to discuss rather simply the onset of instabilities of that kind of perturbations within the frame of a microscopic collisionless theory. Several examples like plane and cylindrical z- and Θ-pinches and plane periodic configurations without space charges or without currents are treated in this way. A special result is, that at least a very wide class of the nonlinear electrostatic waves, found by BERNSTEIN, GREEN and KRUSKAL 1 are unstable.