ON THE PRODUCTION OF SOUND BY LIGHT. 821 



ance in conducting these experiments. When a solution of alum, or 

 bisulphide of carbon, is employed, the loudness of the sound produced 

 by the intermittent beam is very slightly diminished ; but a solution of 

 iodine in bisulphide of carbon cuts off most, but not all, of the audible 

 effect. Even an apparently opaque sheet of hard rubber does not 

 entirely do this. When the sheet of hard rubber was held near the 

 disk interrupter, the rotation of the disk interrupted what was then 

 an invisible beam, which passed over a space of about twelve feet 

 before it reached the lens, which finally concentrated it upon the 

 selenium-cell. A faint but perfectly perceptible musical tone was 

 heard from the telephone connected with the selenium. This could 

 be interrupted at will by placing the hand in the path of the invisible 

 beam. It would be premature, without further experiments, to spec- 

 ulate too much concerning the nature of these invisible rays ; but it is 

 difficult to believe that they can be heat-rays, as the effect is produced 

 through two sheets of hard rubber, containing between them a satu- 

 rated solution of alum. Although effects are produced, as above 

 shown, by forms of radiant energy which are invisible, we have named 

 the apparatus for the production and reproduction of sound in this 

 way, the " photophone," because an ordinary beam of light contains 

 the rays which are operative. 



It is a well-known fact that the molecular disturbance produced 

 in a mass of iron by the magnetizing influence of an intermittent elec- 

 trical current can be observed as sound by placing the ear in close 

 contact with the iron. It occurred to us that the molecular disturb- 

 ance produced in crystalline selenium by the action of an intermittent 

 beam of light should be audible in a similar manner without the aid of 

 a telephone or battery. Many experiments were made to verify this 

 theory, without definite results. The anomalous behavior of the hard- 

 rubber screen suggested the thought of listening to it also. This ex- 

 periment was tried with extraordinary success. I held the sheet in 

 close contact with my ear, while a beam of intermittent light was 

 focused upon it by a lens. A distinct musical note was immediately 

 heard. We found the effect intensified by arranging the sheet of hard- 

 rubber as a diaphragm, and listening though a hearing-tube. We then 

 tried crystalline selenium in the form of a thin disk, and obtained a 

 similar but less intense effect. The other substances which I enumer- 

 ated at the beginning of my address were now successively tried in 

 the form of thin disks, and sounds were obtained from all but carbon 

 and thin glass. We found hard rubber to produce a louder sound 

 than any other substance we tried, excepting antimony, and paper and 

 mica to produce the weakest sounds. On the whole, we feel warranted 

 in announcing as our conclusion that sounds can be produced by the 

 action of a variable light from sxibstances of cdl kinds, tohen in the 

 form of thin diaphragms. We have heard from interrupted sunlight 

 very perceptible musical tunes through tubes of ordinary vulcanized 



