Nikon DIA-ILL  fluorescent microscope dummy filter block for open positions (Diascopeic Illumination), in the cube cassette for the Nikon E400/600/800, TE200/300 fluorescent microscope. The Nikon product number is 96185 with a list price of $38.00, no longer available from Nikon These blocks often are lost and go missing in fluorescent systems. In the Epi-fluorescent attachment instructions, the following Warning appears:

WARNING- Always use four filter blocks (do not leave any of the filter slots empty). Always keep four filter blocks in the cassette holder. If you leave any of the filter slots empty, the instant that the open position is moved into the optical path, light from the light source will reach the eyepiece unfiltered and the user's eyes will be exposed to harmful ultraviolet light, with blindness possibly resulting. (In addition, if there is an empty slot, inertia will cause the filter blocks to slide horizontally when changing filters, creating problems during microscopy.) If fewer than four filter blocks are required for microscopy, use diascopic filter blocks (DIA ILL) as dummies so that there are always four filter blocks installed.