sensing finger holder 1 is movable in the directions of all three coordinates relative to the base part 8 between base part 2 and projection 4 39 of base part 4 an x stop element 9 is operatively connected it has a cutout 10 into which projection 4 39 protrudes at the projection 4 39 and the x stop element 9 an extension spring 11 engages an extension spring 12 is engaged between the x stop element 9 and base part 2 the effective direction of both extension springs 11 and 12 lies in the x coordinate direction by means of the extension spring 11 a stop face 13 on the inside of the x stop element 9 is brought into abutment with the prolongation 4 39 by means of the extension spring 12 base part 2 is pulled against another stop face 14 of the x stop element 9 which lies on the outside of the x stop element 9 in the cutout 10 a stroke length 15 is left free outside of the x stop element 9 opposite stop face 14 a stroke length 16 is left free as shown in fig2 a y stop element 17 comprises a cutout 18 through which projection 4 39 projects an extension spring 19 engages the jacket 6 39 of base part 6 to the y.