1. Method of determining whether a cornea has been modified, said method including the steps of: a. positioning said cornea in the pupil plane of an optical system; b. passing a collimated beam of light through said cornea and a distorted grating to produce, in the image plane of said optical system, intensity maps of said pupil plane and two virtual images located on either side of said pupil plane; c. determining from said images, the wavefront of said light after it has passed through, and been modified by, said cornea; and d. analyzing said modified wavefront for features that identify a modified cornea.