1. In an optical system particularly adapted for use in microscopy and useful for viewing typical objects, said system including a source of illumination, an aperture slit positioned above said source and means including a condenser and objective lens adapted to focus the image of said slit at a Fourier plane, the improvement therewith of apparatus for varying the background of a display associated with said system, comprising: a. a modulator plate positioned at said plane and having different transmittance regions on a surface thereof, with one region normally adapted to recieve said image of said aperture slit, and b. means coupled to said aperture slit to alter said position of said slit image to cause at least a portion thereof to impinge on another region of said modulator, whereby said background intensity is varied according to the portion of said slit image impinging on said another region as compared to the portion of said image impinging on said one region.