Abstract: An optical instrument for use in examining the internal wall of an aperture in a work material comprising a generally tubular housing formed from a material that is opaque to light and having a central axis and first and second ends, the first end having an annular surface that lies in a plane substantially normal to the central axis, a light directing member formed from a transparent material and having a shape in the form of an elongated frustum of a right rectangular pyramid, the member having a longitudinal axis that is coincident with the central axis, a third end having a first rectangular dimension, a fourth end having a second rectangular dimension that is larger than the first dimension, and four faces extending between the third and fourth ends, each face being capable of internally reflecting light passing through the member and incident on the face, the member being mounted within the housing intermediate the first and second ends such that the third end terminates within the housing proximate the