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sound, often using musical examples. He explains that “ difference of color is to the organ
of vision what sharp or flat sounds are to the ear, ” going even further than in his earlier
optical papers to emphasize that “ the parallel between sound and light is so perfect, that
it holds even in the minutest circumstances. ” Thus, the mysterious glowing of phospho-
rescent substances, “ which, once illuminated, preserve their light for some time, though
conveyed into a dark room, ” are sympathetic vibrations made visible.^18 Euler ’ s musical
examples reflect not only the genesis of his theory and its original arguments, but its
justification and popularization in musical terms.