Effective study of in situ stained sections often requires illumination that is difficult to achieve with commonly used research microscopes. One must be able to switch quickly and conveniently from the very lowest to moderate magnifications. At all magnifications contrast due to light scatter must be minimized, so that the weak staining that signifies low gene expression can be observed reliably. For the lowest power objectives (e.g., 1.25x or 2x) many microscopes require that the condenser be removed to illuminate the full field of view. This is not only very inconvenient when switching magnifications, but without a condenser the low numerical aperture of the illuminating light beam results in unwanted contrast due to light scatter. We have devised a simple system that diffusely illuminates the full field of view of the lowest power objective (1.25x) and has high enough numerical aperture for use with the 25x and 40x objectives. A key feature is the use of a large diameter ring light (internal diameter 5.8 cm), placed on the microscope base, to illuminate a large diameter diffuser placed just below the slide.