the Rhine and Danube rivers. He suggested that 10 kg of fluorescein ed thrown into the Danube. Three days later, the characteristic green fluorescence of fluorescein was detected in the waters of the Rhine, demonstrating that these two great rivers were linked. This brief historical perspective should give the reader some idea of the st colorful (no pun intended) past of fluo of rescence. The main events from 1565 to e 1960 are highlighted in Table I. The interested reader can also find more information in References 1-3.