juncture between a sidewall 77 and a bottom wall 79 of the platform sleeve 71 one of each of a pair of glide spheres 81 is nested in a different one of the glide depressions 75 orientation limiting means such as a tilt limiting sphere 83 nested in a spherical tilt limiting depression 85 formed in the arm inner sleeve 49 is provided to provide limited tilting of the tray 73 spacing of the tilt limiting sphere 83 relative to a top wall 91 and a sidewall 92 of the platform sleeve 71 is such that limited axial rotation of the platform sleeve 71 about the arm inner sleeve 49 is provided also the tilt limiting depression 85 has a slightly larger radius than that of the tilt limiting sphere 83 such that the orientation of the tray 73 can be adjusted approximately 15 degrees about a horizontal axis 93 as hereinafter described it is foreseen that a greater or lesser quantity of spheres similar to the first pair of spheres 59 the second pair of spheres 63 the pair of glide spheres 81 and the tilt limiting sphere 83 may be utilized to meet the requirements of a particular application a knurled knob 87 secured to a threaded shaft 89 cooperates with a tapped bore 94 in the top wall 91 to provide releasable securement of both the longitudinal and the rotational displacements of the platform sleeve 71 relative to the arm inner sleeve 49 longitudinal adjustment of the platform sleeve 71 relative to the.