Fig. 1.
Dependence of near ultraviolet repellent response on visible background light. The figure shows the % of cells reversing within 2 s of the near UV stimulus as a function of red background intensity. Points are average values ± extreme values of 2 separate determinations, each based on >40 cells. The inset shows the transmittance spectra of the filter combinations used for stimulus and visible background irradiation. The stimulus and observation light from a 150 W mercury (Hg) lamp and 50 W tungsten Zeiss microscope illuminator, respectively, were combined through a glass beam splitter which allowed passage of 30% of the infrared observation light (Kodak 87A filter) and 30% of the Hg lamp radiation. The combined beam illuminated the cells through the microscope phase condensor. The red background light was provided by an optically collimated 50 W tungsten light source reflected by a quartz beam-splitter mounted below the condenser. Light fluxes at the sample were measured with a Kettering 65 Radiometer and intensities calculated using irradiation areas measured with a calibrated ocular Zeiss micrometer. Light intensity was varied with Kodak neutral density filters. Stimulus light intensity, 1,400 erg cm−2 s−1. Maximum red background intensity (Imax), 7.2 × 104 erg cm−2 s−1. Infrared radiation, >750 nm.