Abstract
The brightness discrimination of larvae of plaice and sole was tested by subjecting them to a choice of moving phototactically to one of two horizontally opposed light sources. At higher light levels, above 100-10-1m-candles, they moved to the brighter of the two lights; below 10-1-10-2 m-candles they moved to the dimmer. Brightness discrimination, expressed as the increment (ΔI) discriminated as a percentage of the dimmer light (I) (the Weber fraction), was of the order of 60-200%.