PHILOSOPHICAL SOCIETY OF WASHINGTON. 143 



THE SPECTROPHONE. 



In a paper read before the American Association for the Ad- 

 vancement of science, last August, I described certain experiments 

 made by Mr. Sumner Tainter aud myself, which had resulted in 

 the construction of a "Photophone," or apparatus for the production 

 of sound by light ;* and it will be my object to-day to describe the 

 progress we have made in the investigation of photophonic phenom- 

 ena since the date of this communication. 



In my Boston paper the discovery was announced, that thin disks 

 of very many different substances emitted sounds when exposed to 

 the action of a rapidly-interrupted beam of sunlight. The great 

 variety of material used in these experiments led me to believe 

 that sonorousness under such circumstances would be found to be 

 a general property of all matter. 



At that time we had failed to obtain audible effects from masses 

 of the various substances which became sonorous in the condition 

 of thin diaphragms, but this failure was explained upon the sup- 

 position that the molecular disturbance produced by the light was 

 chiefly a surface action, and that under the circumstances of the 

 experiments, the vibration had to be transmitted through the mass 

 of the substance in order to affect the ear. It was therefore sup- 

 posed that, if we could lead to the ear, air that was directly in 

 contact with the illuminated surface, louder sounds might be ob- 

 tained, and solid masses be found to be as sonorous as thin dia- 

 phragms. First experiments made to verify this hypothesis pointed 

 towards success. A beam of sunlight was focussed into one end of 

 an open tube, the ear being placed at the other end. Upon interrupt- 

 ing the beam, a clear, musical tone was heard, the pitch depending 

 upon the frequency of the interruption of the light, and the loud- 

 ness upon the material composing the tube. 



At this stage our experiments were interrupted, as circumstances 

 called me to Europe. 



While in Paris a new form of the experiment occurred to my 

 mind, which would not only enable us to investigate the sounds 



* Proceedings of American Association for the Advancement of Science, Aug. 

 27th, 1880 ; see, also, American Journal of Science, vol. xx, p. 305; Journal of 

 the American Electrical Society, vol. iii, p. 3 ; Journal of the Society of Telegraph 

 Engineers and Electricians, vol. ix, p. 404; Annales de Chimie et de Physique, 

 vol. xxi. 



