A substantial body of art exists on the synthesis, classification, and naming of these indigoid dyes. (See for example The Chemistry of Synthetic Dyes by K. Venkataraman, Academic Press, Inc., New York, 1952, pp. 10034045; and Colour Index second edition, 1956, volume 3 published in the United States by the American Association of Textile Chemists and Colourists pp. 3547-3567.) There are a number