Nov. 4, 1880] 



NATURE 



17 



the rays to a focus. Just at the focus is interposed a disk 

 pierced with holes — forty or so in number— arranged in a 

 circle. This disk can be rotated so that the light is inter- 

 rupted from one to five or six hundred times per second. 

 The intermittent beam thus produced is received by a 

 lens T, or a pair of lenses upon a common support, whose 

 function is to render the beam once more parallel, or to 

 concentrate it upon the disk of ebonite placed immediately 

 behind, but not quite touching them. From the disk a 

 tube conveys the sounds to the ear. We may remind 

 our readers here that this apparent direct conversion of 

 light into sound takes place, as Prof. Bell found, in disks 

 of all kinds of substances— hard rubber, zinc, antimony, 

 selenium, ivory, parchment, wood, and that he has lately 

 found that disks of carbon and of thin glass, which he for- 

 merly thought exceptions to this property, do also behave 

 in the same way. We may perhaps remark without im- 

 propriety that it is extremely improbable that the apparent 

 conversion of light into sound is by any means a direct 

 process. It is well known that luminiferous rays, when 

 absorbed at the surface of a medium, warm that surface 

 slightly, and must therefore produce physical and mole- 

 cular actions in its structure. If it can be shown that 

 this warming effect and an intermediate cooling by con- 

 duction can go on with such excessive rapidity that 

 beams of light falling on the surface at intervals less 

 than the hundredth of a second apart produce a discon- 

 tinuous molecular action of alternate expansion and con- 

 traction, then the mysterious property of matter revealed 

 by these experiments is accounted for. 



How-ever this may be, the musical photophone, as 

 represented in Fig. i, produces very distinct sounds, of 

 whose existence and dependence for their production on 

 the light the listener may satisfy himself by cutting off 

 the light at any moment with the little opaque disk fixed 

 on the end of the little iQver just in front of the holes in 

 disk R, and which can be worked by a Morse key like a 

 telegraph instrument, thus producing at will alternate 

 sounds and silences. With this musical photophone 

 sounds have been carried by an interrupted beam of light 

 for a distance exceeding a mile ; there appears, indeed, 

 no reason why a much greater range might not be 

 attained. 



The articulating photophone is that to which hitherto 

 public attention has been most largely directed, and in 

 which a selenium receiver plays a part. Fig. 2 gives in 

 diagram form the essential parts of this arrangement. A 

 mirror M reflects a beam of light as before through a lens 

 L, and (if desired for the purpose of experimentally cutting 

 off the heat-rays) through a cell h containing alum-water, 

 and casts it upon the transmitter 11. This transmitter, 

 shown again in Fig. 5, consists of .a little disk of thin 

 glass, silvered on the front, of about the size of the disk 

 of an ordinary telephone, and mounted in a frame, with 

 a flexible india-rubber tube about sixteen inches long 

 leading to a mouthpiece. A second lens K, interposed in 

 the beam of light after reflection at the little mirror, 

 renders the rays approximately parallel. The general view 

 of the transmitting apparatus given in Fig. 5 enables the 

 relative sizes and positions of the various parts (minus 

 the alum-cell which is omitted) to be seen. The screw 

 adjustments of the support serve to direct the beam of 

 light in the desired direction. 



It may be well to explain once for all how the vibra- 

 tions of the voice can affect the intensity of the reflected 

 beam far away. The lenses are so adjusted that when 

 the mirror D is flat {i.e. when not vibrating) the beam 

 projected from the apparatus to the distant station shall 

 be nearly focussed on the receiving instrument. Owing 

 to the optical difficulties of the problem it is impossible 

 that the focussing can be more than appro.ximate. Now, 

 matters being thus arranged, when the speaker's voice is 

 thrown against the disk B it is set into vibration, becomes 

 alternately bulged out and in, and made slightly convex 



or concave, the degiee of its alteration in form varying 

 with every vibration of the voice. Suppose at any instant 

 — say by a sudden displacement such as takes place when 

 the letter " T " is sounded — the disk becomes considerably 

 convex ; the beam of light will no longer be concentrated 

 upon the receiving instrument, but will cover a much 

 wider area. Of the whole bean, therefore, only a rela- 

 tively small portion will fall upon the receiving instru- 

 ment ; and it is therefore possible to conceive that, if 

 perfectly adjusted, the illumination should be propor- 

 tional to the displacement of the disk, and vary therefore 

 with ever)' vibration with the utmost fidelity. 



The receiver of the articulating photophone is shown 

 on the right-hand side of the diagram (Fig. 2) sketched 

 by Prof. Bell. A mirror of parabolic curve C C serves to 

 concentrate the beam and to reflect it down upon the 

 selenium cell s, which is included in the circuit of a 

 battery p along with a pair of telephones T and T. Here 

 again a general view like that given in Fig. 6 facilitates 

 the comprehension of the principal parts of the apparatus. 

 The sensitive selenium cell is seen in the hollow of the 

 parabolic mirror which is mounted so as to be turned in any 

 desired direction. The battery standing upon the ground 

 furnishes a current which flows through the selenium cell 

 and through the telephones. When a ray of light falls on 

 the selenium— be it for ever so short an instant— the 

 selenium increases in conductivity, and instantly transmits 

 a larger amount of electricity, and the observer with the 

 telephones hears the ray,or the succession of them ; — hears 



yyyykyyyy 



Fig. 4.— Diagra-n to show the action of the Selenium Receiver. 



indeed their every fluctuation in a series of sounds which, 

 since each vibration corresponds to a vibration of the voice 

 of the distant speaker, reproduce the speaker' s tones. 



The great difficulty to be overcome in the use of 

 selenium as a working substance arose from its very 

 high resistance. To reduce this to the smallest possible 

 quantity, and at the same time to use a sufficiently large 

 surface whereon to receive the beam of light, was the 

 problem to be solved before any practical result could be 

 arrived at. After many pieliminary trials with gratings 

 and perforated disks of various kinds. Prof. Bell and Mr. 

 Tainter finally settled upon the ingenious device to be 

 describsd. A number of round brass disks, about two 

 inches in diameter, and a number of mica disks of a 

 diameter slightly less, were piled upon one another so as 

 to form a cylinder about two and a half inches in length. 

 They were clamped together from end to end, the clamp- 

 ing rods also serving to unite the disks of brass electrically 

 in two sets, alternate disks being joined, the ist, 3rd, 5th, 

 &c., being united together, and the 2nd, 4th, 6th, &c., 

 being united in another series. This done, the edges 

 between the brass disks were next filled with seleninum, 

 which was rubbed in at a temperature sufficiently high 

 to reach the melting-point of selenium. After this the 

 selenium was carefully annealed to bring it into the 

 sensitive crystalline state. Then the cell is placed ma 

 lathe and the superfluous selenium is turned off until the 

 ed<:-es of the brass disks are bared. Fig. 3 shows, in section, 

 the construction of such a cell. Prof. Bell has also used 

 cells in which the selenium filled only the alkmatc spaces 

 between disks, the intermediate spaces being occupied by 



