and 56 operating independently in the vertical direction the first probe assembly 48 carries a read or optics probe 50 which aspirates fluid from the tubes into an optics module for quantitative optical determination the second assembly 56 carries adjacent detergent add and aspirate probes 72 and 74 respectively utilized in the wash cycle and a substrate probe to add substrate reagent for chromophore formation also the second assembly 56 carries a diluent quench probe 68 adjacent the substrate probe 66 but for the read cycle the optics probe 50 is spaced from probes 66 and 68 by the distance between rack rows vertical movement of the first probe assembly 50 is accomplished by a first probe motor 42 which drives jack screw 44 a guide rod 46 fixes the position of a first probe assembly 48 in the horizontal plane to permit registration of the probe 50 with the reaction tubes second probe motor 52 controls vertical movement of probes 72 and 74 which are depended from the second probe assembly 56 the probe motor 52 drives a jack screw shown in dashed lines fixed to the bottom of the carriage 20 guide 58 fixes the horizontal position of the second probe assembly 56 the direction of the jack screw drive determines whether the probe assembly ascends or descends probe 72 adds detergent solution to and probe 74 aspirates the same from the tubes referring now to fig3 second probe assembly 56 and its associated mechanisms can be seen here clearly guide 58 and jack screw 60 are.