In work with the microscope, maximum returns in the way of crisp imagery, in detail and in visual comfort, as well, are obtained only when the conditions of illumination are carefully adjusted. Since the eye is a physiological mechanism, special conditions beyond those basic to good photomicrography are necessary for high visual efficiency. The problem of vision through a microscope is discussed and the literature relative to visual acuity is reviewed. Conditions for high visual acuity and visual comfort are outlined.
WILFRID TAYLOR DEMPSTER, "Visual Factors in Microscopy," J. Opt. Soc. Am. 34, 711-717 (1944)