21 passageways are formed in the center block 13 fig1 by drilling or forming a plurality of holes 22 through the block from the top wall to the bottom wall a plurality of grooves 23 are formed in the top surface of the block to fluidly interconnect the holes 22 and a terminal groove 24 is formed in the top surface to fluidly interconnect grooves 23 with the atmosphere at the peripheral wall 21 secured to the center block 13 fig1 and 2 by an adhesive or the like are the cover plates 14 and 15 the cover plates are also translucent or transparent and hexahedron in shape upon securement of the plates to the block with the block 13 sandwiched therebetween the holes and grooves are effectively sealed from the atmosphere with only the terminal groove 24 fluidly open the terminal groove 24 provides a passageway for the substantial evacuation of the air in the holes 22 and grooves 23 upon evacuation of the air a plug unit 25 is inserted into the terminal groove to effectively seal it and the passageways thus preventing the entrance of any air thereto a second embodiment of the invention is disclosed in fig4 5 and indicated generally at 30 this embodiment includes a center block 31 a pair of end plates 32 and 33 and a plug unit 34 the center block 31 sandwiched between the end plates is formed from a transparent or translucent material into a hexahedron and has a bottom wall 36 a top wall 37 a pair of end walls 38 and 39 and a pair of sidewalls 41 and 42 a plurality of holes 43 are formed through the block from end wall to end wall and a groove 44 is.