1. A straight-tube telescope using a mirror or prism, used to visually inspect a cylindrical cavity; such as, the cannon bore of artillery weapons for defects of manufacture and erosion caused by firing.
2. An instrument using optical fibers for the visual inspection of narrow cavities; such as, the bore of a gun.
3. A tool, essentially a tube with a reflecting mirror and an eyepiece, used for inspecting the interior of shafts, well borings, tubing, etc., in order to detect damage.
4. An optical device; such as, a prism or optical fiber which is used to inspect an inaccessible space; for example, an engine cylinder.