of fig4 which would effect a controlled delayed starting of the motor 33 in this manner the duration of the treatment of individual regions of the raceway 2 defined by points along the path 30 could be controlled at times other than the arrest times and at the outermost edge in deviation from the sinusoidal development of the second oscillation as shown in the exemplary embodiment of fig2 so that the treatment duration for the center of the raceway i e its degree of overlap would be minimized in this manner fig5 illustrates a second exemplary embodiment of the present invention for which identical elements have the same reference numerals the right side of the slide 19 as seen in fig5 has disposed in it a roller 42 a spring 44 supported on a locally fixed stop member 43 compare fig2 pushes this roller 42 against a lever 45 which is pivotally mounted in a locally fixed carrier member 47 at a location 46 the lever 45 in turn pushes against a roller 48 which is mounted rotatably at a location 51 within a slot 49 of a further lever 50 the lever 50 has a roller 52 mounted at its upper end as seen in fig5 this.