Prismatic inverters are used to erect the inverted image of indirect ophthalmoscopy. Binocular and monocular indirect ophthalmoscopes can be equally well adapted to yield upright imagery. The inverter fits only to the central area of field that is congruous with the handheld lens. With the lens in place for indirect ophthalmoscopy, the image and the surrounding field yield upright imagery.
Schirmer K, Shea M. Erecting Prism Attachment for Indirect Ophthalmoscopes. Arch Ophthalmol. 1970;83(5):529-532. doi:10.1001/archopht.1970.00990030529001