Fig. 6.

Behavioural response under various permutations of stimulus and background light. Values are averages of duplicate measurements which vary by less than 15%. Experimental conditions as described in Fig. 1 except that reversals were assayed within 3 s of the stimulus, blue and red light were delivered using a tungsten light source and wavelengths were selected by adding or removing filters from the single beam. The Hg light source was blocked in this experiment. ‘Blue’, 380–433 nm (Optics Technology 433 short pass filter+2 mm Plexiglass + two 1–75 Corning infrared absorbing glass), intensity 3.3 × 10⁴ erg cm⁻² s⁻¹. ‘Red’, 530–700 nm (Corning 3–68 + two Corning 1–75), intensity 1.0 × 10⁵ erg cm⁻² s⁻¹. ‘Red+Blue’, The tungsten beam filtered through two Corning 1–75 + 2 mm Plexiglass, intensity 1.5 × 10⁵ erg cm⁻² s⁻¹.