force a rotation of the first element 18 about the first pivot 20 in the case where no lateral drift correction is required the arcuate plate 98 is mounted as shown in fig1 i e with the flat side 108 against the guide and the retaining knob 118 sliding on a flat surface within the arcuate groove 110 as the second element 22 is rotated about the second pivot 66 the guide 66 will thus slide in a channel parallel to the plane of rotation of the second element 22 in that case no rotation will occur about the first pivot 20 and the first element 18 and body 14 will act as an integral member as shown in fig4 the user can also adjust the rotation of the third element 26 about the third pivot 28 by turning a small knob 126 the small knob 126 is keyed onto a threaded shaft not shown which is retained in the third element 26 and threadably received in the second element 22 similarly the user can adjust the rotation of the second adjustment system 32 about the fourth pivot 34 by turning a second small knob 128 the second small knob 128 is keyed onto a second threaded shaft not shown which is similarly retained in the third element 26 and threadably received in the second adjustment system 32 as shown in fig3 each of.