One of the greatest obstacles in learning indirect ophthalmoscopy is the inverted image. Not only must one think in terms of the inverted fundus image but also one must learn to move opposite to the direction which seems natural and which has been learned in direct ophthalmoscopy. It, therefore, seemed logical to design an instrument which would produce an upright image whereby movements would be the same as those used in direct ophthalmoscopy. With this aim in mind, the present piece of equipment was designed. It does not appear that such an instrument has been described heretofore.
The optical portions consist of a pair of penta roof prisms (A and B) whose optical axes are mounted at right angles to each other. These prisms are mounted at a distance apart corresponding to the average interpupillary distance. On one face of each of these prisms is cemented a +3.50 D.
DONALDSON DD. A Binocular Indirect Ophthalmoscope with Upright Fundus Image. AMA Arch Ophthalmol. 1958;60(3):499-501. doi:10.1001/archopht.1958.00940080517026