No. 7.] BELL — THE PIIOTOPHONE. 401 



alogous to the law of electric induction. No effect is produced 

 during the passage of a continuous steady current. It is only at 

 the moment of chansje from a stron2;er to a weaker state, or vice 

 versa, that any audible effect is produced, and the amount of 

 effect is exactly proportional to the amount of variation in the 

 current. It was, therefore, evident that the telephone could only 

 respond to the effect produced in selenium at the moment of 

 change from light to darkness, or vice versa ; and that it would 

 be advisable to intermit the light with great rapidity, so as to 

 produce a succession of changes in theconductivity of the seleninm 

 corresponding in frequency to musical vibrations within the limits 

 of the sense of hearing. For I had often noticed that currents of 

 electricity, so feeble as to produce scarcely any audible effects from 

 a telephone when the circuit was simply opened or closed, caused 

 very perceptible musical sounds when the circuit was rapidly inter- 

 rupted, and that the higher the pitch of sound the more audible 

 was the effect. I was much struck by the idea of producing 

 sound by the action of light in this way. Upon farther consider- 

 ation it appeared to me that all the audible effects obtained from 

 varieties of electricity could also be produced by variations of light 

 acting upon selenium. I saw that the effect could be produced 

 at the extreme distance at which selenium would respond to the 

 action of a luminous body, but that this distance could be inde- 

 finitely increased by the use of a parallel beam of light, so that 

 we could telephone from one place to another without the necessity 

 of a conducting wire between the transmitter and receiver. It 

 was evidently necessary, in order to reduce the idea to practice, 

 to devise an apparatus to be operated by the voice of a speaker, by 

 which variations could be produced in a parallel beam of light, 

 corresponding to the variations in the air produced by the voice. 

 I proposed to pass light through a large number of small orifices, 

 which might be of any convenient shape, but were preferably in 

 the form of slits. Two similarly perforated plates were to be em- 

 ployed. One was to be fixed and the other attached to the centre 

 of a diaphragm actuated by the voice, so that the vibration of 

 the diaphragm would cause the moveable plate to slide to and fro 

 over the surface of the fixed plate, thus alternately enlarging and 

 contracting the free orifices for the passage of light. In this way 

 the voice of a speaker could control the amount of light passed 

 through the perforated plates without completely obstructing its 

 passage. This apparatus was to be placed in the path of a parallel 

 Vol. IX. z No. 7. 



