Blue Day Disc employs etching with aquatint on smooth wove paper to situate a concentrated circular form within a field of cooler blue. Heron's delimitation of the blue 'disc' function as as both shape and chromatic event, advancing and receding as the eye adjusts to the surrounding field. Here the aquatint works with a cadence of blue to create a surface that catches light unevenly, producing a nocturnal richness entirely different in register from the vibrancy of other works by the artist. The sharper the division between colour areas, Heron argued, 'the more intense the colour will be'. This artwork demonstrates Heron's principle against very different chromatic conditions.