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Every physician who makes a practice of including a retinal study in the general physical examination of his patient has probably, like myself, been troubled by the necessity of converting the patient's room into a dark room for a satisfactory study of the eyegrounds. In order to overcome this difficulty it occurred to me that I could convert my ophthalmoscope into a miniature dark room. This was accomplished, as shown in the illustration, by attaching an aluminum eyecup to both sides of the revolving lens system of the instrument. The inside of the cups was painted with a dull finish black paint, which absorbs the side reflections from the wheat grain bulb, so that there is less reflected light to dazzle the eye of both the patient and the examiner. Furthermore, extraneous light is cut off when the eyecups are applied to the eyes of both the examiner and the patient.
Posner C. A MINIATURE DARK ROOM DEVICE FOR OPHTHALMOSCOPES. JAMA. 1935;105(8):586. doi:10.1001/jama.1935.92760340002009b