To appreciate his many sidedness we have bnt to follow the developinent of his life. While his first work was rnainly tnathematical, his seooud was in quite a different field. It consisted in the measureinent of the velooity of propagatiou of sensation by the nerves. To accomplish this he mnst needs have been au anatonaist too. His labors in the line of psychological opties show that he was also a master of psychology. But perhaps it is by his achievements in the reniains of nmsic that he is best known and most celebrated. Iu his book, "The Sensations of Tone, " he solved completely the riddle of nature which had puzzled the woi-ld since the time of Pythagoras. Thns to give a rational numerical explanation of the intricacies of harmony and their effect on the ear, there was need not only of a mathematician, an anatomist, a physicist and a psychologist, bnt also of a musician, all nnited iu one man. Helmholtz was all this and even more.