An instrument for measuring the reflection of an image on the surface of the cornea and other capacities of the eye; used primarily for determining the presence and degree of astigmatism (visual impairment in which part of an image is blurred, due to an irregularity in the curvature of the front surface of the eye, the cornea).

1. An instrument for measuring the reflection of an image on the surface of the cornea and other capacities of the eye, used chiefly for determining the presence and degree of astigmatism.
2. An instrument for measuring the size of a reflected image on the convex surface of the cornea and lens of the eye, by which their curvatures can be determined.