The blue filter is useful for large magnifications (with immersive oils or so) to cut out the long wavelengths and therefore see more sharp.
AND a deep blue filter is supplied. I'm not sure what that's used for. Mytheory is that the filter converts the reddish incandescence of t he electric illuminator to that which simulates daylight.One last thing: Although this microscope has two eyepieces, stereoscopic viewing isn't possible because only onecorresponding objective lens can be used at a time. However I have purchased a stereo microscope, and once it's receivedI hope to post some 3-D microscopic views.