A method and apparatus for observing the wall of a bore hole (22) are disclosed. An optical head (12) is rotated while a sonde (11) is moved up or down within a bore hole (22), and a beam of light from a light source (22) is projected toward the wall of the bore hole (22) via a lens (21), slit (22), half-mirror (19), mirror (15) and lens (14). Light reflected from the bore hole wall is extracted from the half-mirror (19) via a slit (17) and a photoelectric transducer (18) so that the bore hole wall may be continuously observed. By separating the reflected light into spectral components and processing them, color tone logging and other observations can be performed. The orientation of the sonde (11) is sensed by an azimuth finder (23) and rotation gauge (24), and the inclination of the sonde (11) is sensed by a dipmeter (25). Displacement of the sonde accompanying each unit length of up- or -down movement of the sonde to obtain the position being observed by the sonde.