His excursion into the musical realm has also included the personal hearing of many singers so as to determine their fitness for making phonograph records. This proved to be a wonderfully interesting field of investigation, and he has given a great deal of time to it, listening critically to each voice, good, bad, or indifferent, and patiently writing out his criticism in each case. Not only has he heard a large number of singers who have visited the laboratory for the purpose, but he also had a representative scouring Europe for voices several years ago. This man visited the principal cities and towns of Europe and took phonograph records of the voices of the operatic and other prominent singers in each place and shipped them over to Edison, who listened to each one and recorded his opinion in a series of note-books kept for the purpose. He has in the laboratory at Orange nearly two thousand voice records of this kind. All this is done with the object of securing the really best voices in the world. Probably this is the most unique "voice library" in existence. He is very deaf, but has a wonderfully acute inner ear, which, being protected by his deafness from the ordinary sounds of life, will catch minute imperfections that are imperceptible to the person of ordinary hearing. In listening to a voice he uses a peculiarly shaped horn which is held close to the ear, and such is the acuteness of his hearing that he at once distinguishes minute changes of register, extra waves, tremolo, non-periodic vibrations, and other minor defects that detract from the true beauty of vocal sounds. In addition, he can immediately recognize the number of overtones and rate of tremolo, which may afterward be verified by a microscopic examination of a record of the same voice.