• A hand-held optical system for presenting focused stimuli on known retinal areas by direct ophthalmoscopy is described. The optical system provides a time-modulated stimulus beam of variable dimension, retinal illuminance, wavelength, and polarization, and a steady background beam of variable retinal illuminance and wavelength. Both beams enter the patient's eye in maxwellian-view for precise control of the ratio of stimulus to background retinal illuminance. The stimulator-ophthalmoscope allows the examiner to compensate easily for variable fixation. (Arch Ophthalmol 95:1881-1882, 1977)