A keratoscope, sometimes known as Placio's disk, is an ophthalmic instrument for assessing the shape of the anterior surface of the cornea. A series of concentric rings are projected onto the cornea and their reflection viewed by the examiner through a small hole in the centre of the disk. A regular-shaped cornea should show equally-spaced symmetric reflections. If the patient is suffering from astigmatism or from a corneal dystrophy, the rings will be distorted.