Nikon Chroma IGS fluorescent microscope filter cube. Chroma made the filter but it is in an original Nikon filter cube holder. The Chroma number is C9520 and it will fit the NIkon  E400/600/800 and TE200/300 series of Eclipse microscopes. It comes in an original plastic filter cube case which is not pictured.

IGS Cube - In addition to the standard fluorescence cubes, manufacturers may offer a cube for immuno-gold staining. This cube, in place of a dichroic mirror, has a standard half-mirror similar to the kind used in metallurgical brightfield reflected light microscopy. In place of an exciter filter, there is positioned a long pass 420 nanometer barrier filter (to block light below 420) and a polarizing filter oriented to pass light vibrating only east-west perpendicularly to the light entering the cube. In place of a barrier filter on the cube, there is another polarizing filter (serving as an analyzer) which allows only light vibrating north-south to the light path to pass to the eye or detector. The analyzer may be placed in a not quite crossed position with respect to the polarizer. The immuno-gold (or silver) stain shows up quite clearly as it adheres to specific targets being studied.