Abstract: A Wien filter for selecting particles having a given velocity from a beam of charged particles. Such a Wien filter comprises means to maintain an electric field and a magnetic field, which fields extend at right angles to each other and each at right angles to the axis of the particle beam. By providing a gradient in the magnetic field by means of two coils which are present on either side of the said beam and the axes of which are substantially parallel to the electric field and in which the magnetic field strengths produced in the coils are directed substantially opposite to each other, both a focus of the particle beam in a more favorable place is obtained and a stigmatic reproduction is effected so that velocity separation is considerably simplified.