Pictured is a Nikon Blue Color Correction Filter. It is necessary to be used on halogen illuminated microscopes to bring the yellow tint light back towards a natural day light blue. Very important when trying to identity stained blood and bone marrow cells. The filter is in good condition and 45mm in diameter. It fits all of Nikon microscopes manufactured since the early 90's as well as Olympus scopes of the same era. Will also work on Zeiss and Leitz as long as the field lens is 45mm. Thanks for looking.