referring to fig1 - 5 a rotor 1 is supported for limited rotation by a base 2 . through holes 3 are provided in the rotor 1 to allow access to base mounting holes 4 by which screws may mount the base 2 to a support structure . in this embodiment three parallel arcuate tracks 5 , 6 and 7 engage four balls 8 with tracks 5 and 6 having opposed arcuate surfaces as shown in fig1 and track 7 having an arcuate surface as shown in fig1 . in this arrangement the tracks 5 and 6 comprise a arcuate v - groove 9 in base 2 laterally locating the associated balls 8 and in engagement with a flat matching arcuate surface 10 defined by rotor 1 and track 7 has opposed arcuate v - grooves 11 laterally locating the associated ball 8 and in engagement with each other freedom of movement along these grooves as hereinafter described . three of the balls 8 are arranged with two spaced apart in track 5 and one in track 6 all three being located by cages 15 while the fourth ball 8 is located in track 7 and is free to move along track 7 to find a position in which all four balls touch and support the rotor 1 on the base 2 . by this means a smooth sensitive stable high load non - eccentric stage with a movement range typically of about 10 ° about the axis of rotation of the rotor is provided . fig6 - 8 refer to an alternative two track 17 arrangement which may be substituted for the three track arrangement of fig1 - 5 . in fig6 - 8 one track is as illustrated in fig1 while the other track is as illustrated in fig1 . the arcuate flat surface 10 on one side of one of tracks permits the required rotation of the rotor about an axis normal to the axis or rotation 12 to achieve the necessary four point contact of the balls with the rotor 1 and base 2 . an actuating mechanism 18 ( motorized or manual ) is operatively connected to rotor 1 to control movement about the axis 12 . a spring 14 engages the rotor 1 and base 2 to bias these together in engagement with the balls 8 . this four ball 8 arrangement is an example of the track configuration which may be utilized in the four ball embodiment of fig9 and 10 which illustrates a preferred four ball embodiment allowing an angular travel of the rotor 1 about the axis of rotation 12 typically of about 10 ° total under the spring 14 biased control of micrometer 13 or other actuator and , optionally , linear travel under the spring 19 biased control of micrometer 16 or other actuator . the spring 25 located in base 2 engages a pin 20 fast with the rotor 1 and moveable in an opening 21 against the bias of spring 25 by micrometer 13 to pivot the rotor 1 , relative to the base , about axis 12 . the spring 19 engages rotor 1 and a support 22 for micrometer 16 to allow linear movement of the rotor 1 relative to the base 2 along axis 12 against the bias of the spring 19 by operation of the micrometer 16 . in the preferred form the arcuate grooves are either or both grooves shown in fig1 and 14 . to restrain longitudinal movement , a stop / micrometer support 22 may be placed at any convenient point against which the rotor may be pre - loaded . this “ stop ” may also be a ball placed up against a plate attached to the base , and free to rotate with the rotor longitudinal movement may even be locked by having one of the balls captivated between 2 transaxial v - grooves , one in the rotor and one in the base 2 . fig1 - 14 illustrate four groove configurations of which those of fig1 and 13 have already been described above . in addition fig1 illustrates a captivation ( 1 ) in which two flat opposed arcuate surfaces 10 having surface defining side walls oppose one another with a ball 8 between the surfaces and fig1 illustrates a captivation ( 4 ) comprising two flat arcuate opposed surfaces 10 without surface defining side walls . 1 ) one ball with captivation 3 ( fig1 ), and three balls 8 with captivation 1 , 2 or 4 ( fig1 , 12 or 14 ); 2 ) no longitudinal adjustment , with an external stop . here four balls are used with any combination of captivations 1 and 4 ( fig1 and 14 ); and 3 ) longitudinal adjustment is provided by micrometer or motor screw with four balls with any combination of captivations 1 and 4 ( fig1 and 14 ). captivation modules may be externally added rather than machined into the base 2 and rotor 1 , to provide one or more of the grooves . additionally , external bearing housing segments may be transplanted into receivers to form captivation style 3 ( fig1 ), with three of those segments being longitudinally free to find seating for the bearings , then i . e . bonded into those seating places as found . referring now to the three ball embodiments , reference is first made to fig1 - 18 in which a rotor 1 is supported for limited rotation by a base 2 . through holes 3 are provided in the rotor 1 to allow access to base mounting holes 4 by which screws may mount the base 2 to a support structure . in this embodiment two parallel arcuate tracks 5 and 6 engage three balls 8 with tracks 5 and 6 having opposed arcuate surfaces as shown in fig1 and 13 respectively . the balls 8 two spaced apart in track 5 ( fig1 ) and one located centrally in track 6 ( fig1 ) are located by cages ( 15 ). by this means a smooth sensitive stable non - eccentric stage with a movement range typically of about 10 ° about the axis of rotation 12 of the rotor 1 is provided . now referring to fig1 and 20 a preferred 3 ball embodiment allowing an angular travel of the rotor 1 about the axis of rotation 12 typically of about 10 ° total under the spring 14 biased control of micrometer 13 or other actuator . the spring 14 located in base 2 engages a pin 20 fast with the rotor 1 and moveable in an opening 21 against the bias of spring 14 by micrometer 13 to pivot the rotor 1 , relative to the base , about axis 12 . the goniometer of fig1 and 20 may be embodied in the same arrangements as described with reference to fig1 - 14 above . it will be appreciated that in tracks having an arcuate v - groove opposing flat arcuate surface the arcuate v - groove may be located in either the base 2 and the arcuate flat surface on or in the rotor 1 or vise - versa . except where freedom of movement of a ball 8 along its associated arcuate groove is required the balls 8 are located by retaining cages ( i . e . cage 15 in fig1 ). such retaining cages will be well known to those in the bearing arts . the goniometer of fig2 and 22 has four balls 8 unconstrained by grooves or tracks . the balls 8 are retained by a cage 15 , common to all four balls 8 , to maintain ball pairs separated at desired spacings . as with the above described embodiments , this embodiment has a rotor 1 supported for limited rotation , about axis 12 , on a base 2 . as with fig1 and 20 a stop pin 20 , in an opening 21 , maybe provided to facilitate manual or motorized actuation . a stop 22 ( ball , roller or friction slide ) supported by a stop support 23 is provided and a spring 24 is positioned to urge the rotor 1 against the stop support 23 by way of the stop 22 . a possible variation permits the use of the track shown in fig1 . it will be appreciated that the embodiment of fig2 and 22 is quite similar to the arrangement suggested by fig1 which illustrates a captivation comprising two opposed flat arcuate surfaces without surface defining side walls . to restrain longitudinal movement , a stop may be placed at any convenient point against the rotor . this “ stop ” may be a ball placed up against a plate attached to the base , and free to rotate with the rotor . as used in this application “ balls ” shall be construed to include “ rollers ” in cases where the arcuate flat surfaces as shown in fig1 and 14 are used .