Placido disk is a device used to qualitatively examine corneal irregularities and astigmatism.
There is a small convex lens on the back side to enable close examination without needing to accommodate.
It is made of painted brass. There was originally a wooden handle; now absent.

Placido A: Novo instrumento per analyse immediate das irregularitades de curvatura da cornea. Periodico Oftalmol Practica 1880; 6:44-49.