an apparatus for inspecting the top and bottom surfaces of a tape or sheet has a device to magnify the surface of the tape or sheet , with its objective lens disposed to face the top surface of the tape or sheet . the apparatus also has an optical transmittion system that transmits an image of the bottom surface of the tape or sheet through a focusing unit and four reflectors , each of which is adapted to bend the path of light rays 90 degrees . of the four reflectors , one that faces the objective lens is retractable from the light - ray path in the optical transmission system . the reflector facing the objective lens may be a half - coated mirror . fixed in the light - ray path in the optical transmission system , the half - coated mirror is not retractable .