Optical instrument employing a truncated prism having its distal end disposed proximate a planar end of an opaque tubular housing

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, said first end having an annular surface that lies in a plane substantially normal to said central axis;
• a first lens mounted within said housing intermediate said first and second ends, said first lens including opposed first planar and convex surfaces and being capable of focusing and magnifying an image incident on said first planar surface;
• a light directing member formed from a transparent material and having a longitudinal axis that is coincident with said central axis, said member including a reflectig portion having a shape in the form of an elongated frustum of a right rectangular pyramid and a mounting portion that is disposed in an abutting relationship relative to said first planar surface, said reflecting portion including a third end having a first rectangular dimension, a fourth end having a second rectangular dimension that is larger than said first dimension, and four faces extending between said third and fourth ends, each said face being capable of internally reflecting light passing through said member and incident on said face, said mounting portion being mounted within said housing intermediate said first and second ends such that said third end terminates within said housing proximate said first end;
• a second lens mounted within said housing intermediate said second end and said first lens, said second lens including a second planar surface disposed in a facing relationship relative to said first convex surface and an opposed second convex surface and being capable of focusing and magnifying an image passing through said first lens;
• a generally tubular spacer disposed between said first convex surface and said second planar surface for maintaining a predetermined spacing between said lenses;
• and
• a generally tubular retaining ring disposed between said second convex surface and said second end, and serving to maintain said second lens within said housing, whereby when said first end is placed against said work material with said axes in substantial alignment with the center of said aperture, and said second end is placed in front of the eye of a viewer, ambient light reflected off said internal wall passes through said third end and is reflected internally by said faces so as to provide images representative of corresponding portions of said internal wall, and whereby said first and second lenses serve to focus and magnify said images so as to permit the viewer to observe the physical appearance of said internal wall.