Abstract: An apparatus for measuring the distance between two locations on a surface within a cavity includes a sheath which is inserted into the cavity and a scope which extends through the sheath. Through the scope, one from a remote location may observe the surface as well as reference marks which appear within the field of view on each side of the optical axis. These reference marks are aligned with opposite sides of that which is to be measured on the surface. In addition, the apparatus includes two point sources of light located along opposite sides of the scope with each source being located behind a lens that is presented toward the surface which is observed within the field of view. The lenses project their sources of light into the field of view as rays which come into focus on the surface.