Abstract: A method and an arrangement for detecting the position of the plane XY of an object, which plane XY is to be scanned, and for its positioning in the focal plane X′Y′ of a laser scanner, preferably for a laser scanning microscope. According to the disclosure, it is provided in a method of the type described above that, after a rough orientation of the object carried out by placing on an object holder, a laser beam is directed successively in time to at least three different points P1, P2 . . . Pn located in the scan plane XY of the object and, in doing so, each of the reflections proceeding from the points P1, P2 . . .

Abstract: A process for the preparation of flameproofed polystyrene whicha) contains halohydrocarbons as flameproofing agents and which additionally contains one or more agents from the group ofb) synergists for the flameproofing agent,c) stabilizers,d) antioxidants ande) additivesby mixing polystyrene with one or more of agents a) to e) in a plurality of stages, wherein in a first stage the flameproofing agent a) and the entire amount or a proportion of the stabilizers c) are premixed at up to 100.degree. C. in a mixing apparatus and this premix is then, in a second stage, mixed with the other agents, in a continuously operated single-screw or twin-screw extruder or a kneader of conventional construction, in such a manner that, viewed in the conveying direction of the extruder, first the premix, with or without other agents, is continuously fed in and thereafter, in the further mixing zones, a melt of a polystyrene resin, which has been substantially freed from monomers and solvents and is at from 200.degree. to 250.