a wash and removal step to separate bound reacted from unbound unreacted reagent marker antibody and the like if the slide staining rack has capacity for 50 slides the staining rack can be used to stain any number of slides up to 50 if the number of slides present in the slide staining rack is substantially less than 50 it may be desirable to insert a filler block not shown into the rack to act as a spacer for missing slides a filler block can be made of any suitable material such as plastic and can be formed so as to interconnect with other filler blocks in order to make a spacer of any desired thickness the spacing can also be adjusted by constructing the spacing shims in a manner such that they are attached to the staining rack by a frictional fit permitting them to be removed by simply pulling spacing shims from the rack this makes it possible to readily adjust the number of spaces in the rack with the number of slides which are to be processed and also eliminates any necessity for inserting a filler block into the staining rack each cresent shaped element can be formed either as an independant member connected by suitable means such as spot welding to the base of the spacing shim or can be made by cutting and shaping one side of the spacing shim base alternatively other means such as coil springs can be attached to flat base members for the purpose of making a compressible shim rather than being attached to opposite ends of the top member of.