PURPOSE: To obtain a uniform visual field when the surface of an object having curvature such as the cornea is observed by disposing a double refractive prism in the position conjugate with the central point of curvature of an object to be inspected having a spherical surface with respect to an object lens or making said disposition possible.

CONSTITUTION: A Wollaston prism 5 is disposed at the conjugate point C' with the center C of curvature of an object surface 17 with respect to an objective lens 6 in such a way that the branching points of an ordinary ray (o) and an extraordinary ray (e) by the prism 5 coincide. Consequently, the two rays (o) and (e) out from the prism 5 are made incident vertically to the surface 17 so as to be concentrated at the center C of the surface 17 by the lens 6 and are reflected by tracing the same route reversely. Then the differences in brightness and darkness in the visual field for observation are eliminated, hues are made constant and the differential interference image of the surface 17 is observed satisfactorily and accurately within the uniform visual field.