i6 



NA TURE 



\_Nov. 4, 1880 



mental conception dates from 1S78, when in lecturing phone, however, outgrew the particular electrical combi- 

 before the Royal Institution Prof. Bell announced the nation that suggested it ; for not the least of the remarkable 

 possibility of hearing a shadow fall upon a piece of points in this research is the discovery that audible vibra- 

 seleniura included in a telephone circuit. The photo- | tions are set up in thin disks of almost every kind of 



Tig. I.— The Musical Teleph^ 



material by merely throwing upon them an intermittent 

 light. Hence in theory, if not in practice, the receiver 

 may be reduced to the divine simplicity of a mere disk of I 



Fig. 2. — Theoietical Di 



vulcanite or of zinc, on one side of which the vibrating 

 beam of light falls, and at the other side of which the 

 hearer listens. 



■ n:fimM.ffn;njiJi'j]iir!fl n n iv,n n n'ra vm wmndmw^ ^ 



Fig. 3— Section of the Selenium Receiver, shown at s in Fig. 2. 



With the photophone, however, as with the telephone, 

 there are instruments of different degrees of perfection. 

 The original imperfect musical telephone of Philip Reis 

 could op.iy transmit musical toncs^ because it worked by 



rapid abrupt interruptions of the electric current ; while 



the articulating telephone of Graham Bell was able to 



transmit speech, since by its essential construction it was 



able to send undulating cun-ents to the distant 



receiving station. 



We may in like manner classify the forms 

 of photophone under two heads, as (i) articu- 

 lating photophones, and (2) musical photo- 

 phones ; the former being able to transmit 

 speech because they work by beams of light 

 whose intensity can vary in undulatory fluc- 

 tuations, like those of vocal tones ; the latter 

 being able to transmit simple musical tones 

 only, since they work by mere interruptions 

 of a fixed beam of light. 



Up to the present time. Prof. Bell informs 

 us, the simple receiving disk of ebonite or 

 hard rubber has only ser\-ed for a musical 

 photophone : the reproduction of the tones of 

 the voice by its means has not yet been demon- 

 strated in practice — at least to his satisfaction. For 

 while it produces unmistakable musical tones by the 

 direct action of an intermittent light, in the experiments 

 made hitherto with articulate speech the instru- 

 ments have by necessity been so near to one 

 another that the voice of the speaker was audible 

 through the air. Under these circumstances 

 it is extremely difficult to say whether the 

 sounds that are heard proceed from the dia- 

 phragm^ or whether they merely came through 

 the air to the ear, and if they come from the 

 diaphragm, whether they are really the result 

 of the varying light, and not mere sound vibra- 

 tions taken up by the disk from the speaker's 

 voice crossing the air. Prof Bell hopes soon 

 to settle this point, however, by an appeal to 

 experiment on a larger scale with the receiving 

 and transmitting instruments at greater dis- 

 tances apart, and with glass windows in between ' 

 to shut off all sounds. ' 



In Fig. I we illustrate the simple musical i 

 photophone of Bell and Tainter. It might ' 

 perhaps be described without injustice as an 

 optical siren, producing sounds from inter- 

 mittent beams of light, as the siren of Cagniard 

 de Latour produces them from intermittent puffs of air. 

 A beam of light from the sun or from a powerful artificial 

 source, such as an electric lamp, falls upon a mirror M, 

 and is reflected through a large lens L, which concentrates 



