through hole 25 and has an outer diameter larger than the inner diameter of the cylindrical passage 12 the upper and lower ends of the adjusting knob 20 are each formed with a neck 26 having a smaller diameter than the cylindrical passage 12 so that the adjusting knob 20 can be fitted in the recess 11 with its necks 26 fitted into the cylindrical passage 12 the outer surface of the adjusting knob 20 has a plurality of longitudinal raised lines for facilitating the rotation thereof the arm 22 is provided with a soft pad 23 see fig2 c at the bottom and a downwardly depending screw 21 at the corner the screw 21 of the arm 22 is fitted into the cylindrical passage 12 of the rectangular frame 10 to engage with the threaded hole 25 of the adjusting knob 20 in assembly the outer cover is engaged with the inner cover to form the rectangular frame 10 with the anti radiation screen 30 fitted therebetween then the two adjusting knobs 20 are fitted into the recesses 11 of the rectangular frame 10 thereafter the two arms 22 are mounted on two upper corners of the rectangular frame 10 with the screws 21 extending through the cylindrical passages 12 to engage with the adjusting knobs 20 hence when the adjusting knobs 20 is rotated the arms 22 will be moved up or down depending on the direction that the adjusting knobs 20 are turned as shown in fig1.