The transmission losses occurring in microscope optical systems are discussed and their effect on the brightness of both visual field and projected images is estimated. When the conditions requisite for maximum efficiency are complied with a simple formula enables the highest possible screen or visual field illumination to be readily calculated from the known constants of the illuminant and the optical components. It is shown that the production of large, well illuminated, projected images by means of a microscope is subject to limitations for which the only solution is the provision of illuminants having a higher intrinsic brilliance than any at present available.A limitation on screen brightness may in some instances have to be imposed by the need to limit the energy in traversing the specimen.