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thie details of the experiment soon appeared in the columns of Nature, from Harry Napier Draper 8 and Lieut. M. L. Sale,9 which were answered in the next number by Willoughby Smith.10

Sale and Draper were soon able to corroborate the statements that had been made by Willoughby Smith.

Sale presented his researches to the Royal Society on the 8th of May, 1873, and in the following November, Draper 12 presented his results to the Royal Irish Academy in the shape of a joint paper by himself and Richard J. Moss.

Draper and Moss gave in their paper an admirable summary of the condition of our knowledge regarding selenium at that time. They confirmed Hittorff's observation that the temperature of minimum resistance of granular selenium was somewhere about 210° C., and that at 217° C. (the fusing point), the resistance guddenly increased. They carried the temperature to a still higher point than Hittorff had done, and found that the resistance again diminished, reaching a second minimum at 250° C.

During the course of their experiments they produced a variety of granular selenium not different in appearance from other specimens but having different electrical properties. In this form the resistance became greater instead of less when the temperature was raised.

They also used thin plates of selenium instead of the cylindrical bars formerly employed, and found great advantage from the increased sensitiveness of the former to light.

Sale found upon cxposing selenium to the action of the solar spectrum that the maximum effect was produced just at or outside the extreme edge of the red end of the spectrum at a point nearly coincident with the maximum of the heat rays, thus rendering it uncertain whether the effect was due to light or to radiant heat.

In the winter of 1873–4 the Earl of Rosse 13 attempted to de

& Nature, Vol. VII, p. 340, Mch. 6, 1873. 'Ibid. 10 Nature, Vol. VII, p. 361, Meh. 13, 1873.

11 Proc. Roy. Soc., Vol. XXI, p. 283; see also Pogg. Ann., Vol. CL, p. 333; Phil. Mag. (lth ser.), Vol. XLVII, p. 216; Nature, Vol. VIII, p. 134.

12 Proc. Roy. Irish Acad. (2nd ser.), Nov. 10, 1873, Vol. I, p. 529; see also a communication from Richard J. Moss to Nature, Aug. 12, 1875, Vol. XII, p. 291; being an answer to a letter from J. E. II. Gordon, upon the “Anomalous behavior of Selepjum," published in that journal on the 8th of July, 1875; sce Vol. XII, p. 187.

13 Phil. Mag.(4th ser.), March, 1874, Vol. XLVII, p. 161; see, also, Am. Journ. of Sci. and Arts (3rd ser.), Vol. VII, p. 512.

cide this question by comparing the selenium effects with the indications of the thermopile. He exposed selenium to the action of non-luminous radiations from hot bodies, but could produce no effect; whereas, a thermopile under similar conditions gave abundant indications of a current. - IIe also cut off the heat rays of low refrangibility from luminous bodies by the interposition of glass and alum between the selenium and the source of light without materially affecting the result; but when the thermopile was employed the greater portion of the heateffect was cut off.

Later, Prof. W. G. Adams, 14 of Kings College, took up the question, and his experiments seemed to prove conclusively that the action was due principally, if not entirely, to those rays of the spectrum which were luminous, and that the ultra red or the ultraviolet rays had little or no effect.

This conclusion was supported by the marked effect produced by the light of the moon, and by the apparent insensitiveness of selenium to rays passed through a solution of iodine in bisulphide of carbon. He found that the maximum effect was produced by the greenish-yellow rays, and showed that the intensity of the action depended upon the illuminating power of the light, being directly as the square root of that illuminating power.

Professor Adams and Mr. R. E. Day 15 continued these researches; and, among other interesting and suggestive results, discovered that light produces in selenium an electromotive force without the aid of a battery.

· The most sensitive variety of selenium that has yet been produced was obtained in Germany, by Dr. Werner Siemens, by continued heating for some hours at a temperature of 210° C., followed by extremely slow cooling.

Dr. C. W. Siemens, 16 in a lecture delivered before the Royal Institution of Great Britain, on the 18th of February, 1876, stated that his brother's modification of selenium was so sensitive to

14 Proc. Roy. Soc., June 17, 1875, Vol. XXIII, p. 535; see, also, Proc. Roy. Soc., Jan. 6, 1876, Vol. XXIV, p. 103. Nature, Jan. 20, 1876, Vol. XIII, p. 238. Nature, Mar. 23, 187 Vol. XIII, p. 419. Scient. Amer. Supplement, June 1876, Vol. I, p. 351.

15 Proc. Roy. Soc., June 15, 1876, Vol. XIV, p. 113.

16 Proc. Roy. Iust. Gt. Brit., Feb. 18, 1876, Vol. VIII, p. GS; fee also, Nature, Vol. XIII, p. 407; Scient. Amer. Supplement, Apr. 1, 1876, Vol. I, p. 222; Scient. Amer. Sup. plement, June 10, 1876, Vol. I, p. 375.

light that its conductivity was fifteen times as great in sunlight as it was in the dark.

In Werner Siemens'17 experiments special arrangements were made for reducing the resistance of the selenium.

For this purpose two fine platinum wires were coiled into a double flat spiral and were laid upon a plate of mica, so that they did not come into contact with one another. A drop of melted selenium was then placed upon the platinum wire arrangement, and a second sheet of mica was pressed upon the selenium so as to cause it to spread out and fill the spaces between the wires. Each cell was about the size of a silver dime. The cells were then placed in a paraffine bath and annealed.

Siemens devised other arrangements of apparatus for reducing the resistance. In the form known as “Siemens' Grating," the two wires, instead of being coiled together, were arranged in zigzag shape, forming a sort of platinum gridiron.

This was treated in the same way as the spiral arrangement. Another form of cell consisted of a sort of lattice-work or basketwork of platinum wires arranged upon a perforated mica plate, the wires interlacing with one another, and with the mica plate so as to make metallic contact only with alternate wires. He also found that iron and copper might be employed, instead of platinum.

Without dwelling further upon the researches of others I may say that all observations concerning the effect of light upon the conductivity of selenium had been made by means of the galvanometer, but it occurred to me that the telephone, from its extreme sensitiveness to electrical influences, might be substituted with advantage. Upon consideration of the subject, however, I saw that the experiments could not be conducted in the ordinary way, for the following reason : The law of audibility of the telephone is precisely analogous to the law of electric induction. No effect is produced during the passage of a continuous and steady current. It is only at the moment of change from a stronger 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.

17 Monatsbericht der Kön. preuss. Akad. der Wissenschaften zu Berlin for 1875, p. 250; Phil. Mag., Nov. 1875 (1th ser.), Vol. L, p. 416; Nature, Dec. 9, 1875, Vol. XIII, p. 112; Vonatsber. Berl. Akad., Feb. 17, 1876; Pogg. Ann., Vol. CLIX, p. 117; Monatsb. Berl. Akad., June 7, 1877; Pogg. Ann., 1877, Vol. II, p. 521.

It was, therefore, evident that the telephone could only respond to the effect produced in selenium at the moment of change from light towards 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 the conductivity of the selenium 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 hardly to produce any audible effects from a telephone when the circuit was simply opened and closed, caused very perceptible musical sounds when the circuit was rapidly interrupted; and that the higher the pitch of the sound the more audible was the effect. I was much struck by the idea of producing sound in this way by the action of light.

I proposed to pass a bright light through one of the orifices in a perforated screen consisting of a circular disk or wheel with holes near the circumference. Upon rapidly rotating the disk an intermittent beam of light would fall upon the selenium, and a musical tone should be produced from the telephone, the pitch of which would depend upon the rapidity of the rotation of the disk.

Upon further consideration, it appeared to me that all the audible effects obtained from variations of electricity could also be produced by variations of light, acting upon selenium. I saw that the effect could not only be produced at the extreme distance at which selenium would normally respond to the action of a luminous body, but that this distance could be indefinitely increased by the use of a parallel beam of light, so that we might 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 this 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 perforated plate containing an immense number of small orifices.

Two similarly perforated plates were to be employed. One was to be fixed and the other to be attached to the centre of a diaphragm actuated by the voice; so that the vibration of the diaphragm would cause the movable 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 beam of light, and the undulatory beam emerging from the apparatus could be received at some distant place upon a lens, or other apparatus by means of which it could be condensed upon a sensitive piece of selenium placed in a local circuit, withi a telephone and galvanic battery.

The variations in the light produced by the voice of the speaker should cause corresponding variations in the electrical resistance of the selenium at the distant place, and the telephone in circuit with the selenium should reproduce audibly the tones and articulations of the speaker's voice.

I obtained some selenium for the purpose of trying the apparatus described, but found upon experiment that its resistance was almost infinitely greater than that of any telephone that had been constructed ; and I was therefore unable at that time to obtain audible effects in the way desired. I believed, however, that this obstacle could be overcome by devising mechanical arrangements for reducing the resistance of the selenium, and by constructing special telephones for the purpose.

I felt so much confidence in this that in a lecture delivered before the Royal Institution of Great Britain, on the 17th of May, 1878, I announced the possibility of liearing a shadow by means of interrupting the action of light upon selenium. A few days afterwards my ideas upon this subject received a fresh impetus by the announcement made by Mr. Willoughby Smith, 18 before the Society of Telegraph Engineers, that he had heard the action of a ray of light falling upon a bar of crystalline selenium by listening to a telephone in circuit with it.

It is not unlikely that the publicity given to the speaking telephone during the last few years may have suggested to many minds, in different parts of the world, somewhat similar ideas to my own; indeed, it has recently come to my knowledge that a writer (J. F. W.,19 of Kew), on the 13th of June, 1878, asked

18 Sec Journ. of Soc. of Teleg. Engin., May 23, 1978, VII, 281.
19 Nature, Vol. XVIII, p. 169.

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