Zeiss four-mirror gonioprisms tend to lose their silver coating easily. Their life can be prolonged by painting with clear nail polish or resilvering. When resilvering was carried out locally, tiny particles of silver invisible to the naked eye were left on the corneal surface and transferred to the patient's corneal epithelium. It is urged that if resilvering is carried out locally, the corneal surface of the prism should be carefully examined with a slit lamp immediately upon return of the instrument. This precaution appears to be unnecessary if the resilvering is done by Zeiss.
Grady FJ. A Rare Complication of Gonioscopy With the Zeiss Gonioprism. Arch Ophthalmol. 1972;88(4):432-433. doi:10.1001/archopht.1972.01000030434018