it is preferred however that stops 34 be driven with computer signal encoding which may be accomplished as follows each of stops 34 extends through plate 40 which supports drive mechanism 42 plate 40 is secured to collet 12 with screws 46 and each of stops 34 are terminated with a gear 44 in mesh with each of gears 44 is an idler gear 48 which in turn is driven by a motor m the operation of motors m may be controlled by any suitable means e g encoders e geared to idlers 48 once a block b is adjusted for desired prism angle by seating against stops 34 the block is clamped for lens l surfacing by closing fingers 16 of collet 12 with sleeve 24 with lens l tilted relative to collet axis a a by the amount of block b adjustment a prism angle corresponding to the resulting tilt of lens l may be generated with conventional on axis lens surfacing apparatus for example a cupped lens grinding tool t may be swept across lens l in a curved path p having its center of curvature c 1 on axis a a the radius of curvature r of path p will determine the generated surface radius in one meridian thereof and the angle of presentation of abrading face 50 of tool t will establish the radius of generated curvature in a meridian normal to the one meridian those interested in details of the above technique for generating spherical and toric surface curvatures may refer to u s pat nos 3 118 198 and 3 152 427 lens generating operations using preformed tools as in u s pat nos 3 117 396 and 3 624 969 may also.