by precision ball bearing screws 41 42 which are rotated by stepping control motors 43 44 for power and position control the entire x y axis positioner assembly is supported on a z axis positioner assembly 40 which is driven in the vertical direction as hereinafter described the specimen stage 35 protrudes out from the y axis slide positioner sliding member 36 and has a wide opening 45 in its center shown occupied by filler 52 in fig1 and 6 upon y axis motion to place the specimen slide 12 between condenser lens 9 and objective lens 10 the filler 52 remains behind and the opening 45 exposes the maximum area of the specimen slide transparent surface to illumination for optical observation the specimen stage 35 has an edge guide 46 and an end stop 27 which assist to constrain and position the specimen slide 12 during loading onto the specimen stage 35 the end stop 27 may have a slight inverted bevel to help retain and push the specimen slide 12 downward against the stage support surfaces during transfer to the specimen stage 35 the specimen slide 12 is guided into and clamped against the edge guide 46 and retained against end stop 27 by an elongated bow shaped spring 48 which is mounted by slidably projecting through horizontal bore holes in posts 49 projecting upward from the protruding ends of specimen stage 35 as a specimen slide 12 is driven by push bar 19 the leading corner and edge engage the bow spring 48 at.