Various types of light sources were used during the course of the present experiments; an Osram quartz-iodine lamp operated at d-c 15 volts and approximately 150 watts was used in the later staage of the present study as a source of near-visible ultraviolet light. Quartz lenses L1 and L2 (Fig. 1) were used to condense the light on a 1-3-mm portion of the nerve. Optical filter F1, used to absorb visible light, was either a Corning glass filter (CS 7-83) or a Bausch and Lomb interference filter for 365 m,. Visible light emitted by the nerve was detected with a photomultiplier tube (RCA 4463) at a right angle to the direction of the incident light. Filter F2, used to absorb the incident light scattered by the nerve, was either a Corning filter CS 3-72 (transmitting visible light longer than 430 m,u in wavelength) or a Corning filter CS 5-75 (transmitting a narrow spectrum around