SPECTROPHONE and PHOTOPHONE or RADIOPHONE. The action of these instruments depends upon the phenomena of radiant energy which within a few years have been observed by different physicists, but whose practical results, if the time has arrived when the term may be used, have been principally accomplished by prof. Alex ander Graham Bell, the inventor of the speaking telephone iu common use, in conjunc tion with Mr. Sumner Taiuter. The invention of the photophone arose from the employ ment by Willoughby Smith of selenium as a resistance medium in testing submarine cables. It was found that the resistance of selenium to the galvanic current varied con siderably, and the discovery was made that this was caused by the action of light, which lessened the resistance. When selenium is exposed to the action of the solar spectrum, the maximum effect is produced, according to Sale, just outside of the red end of the spectrum, in a point nearly coincident with the maximum of the heat rays (see Radiation in article HEAT, ante), but according to Adams it is produced in the greenish-yellow or most luminous part of the spectrum, and he moreover found that selenium was sensitive to the cold light of the moon. E. W. Siemens discovered that heat and light produced opposite effects upon some extremely sensitive varieties of selenium. In some of his experiments the resistance on exposure to light was only one-fifteenth of what it was in the dark. It occurred to prof. Bell to substitute the telephone for the galvanometer hitherto used in these experiments on accounted its great sensitiveness to electrical influ ences; but in doing so it was necessary to vary the action of light so that the inter missions front light to darkness should be sudden, in order to produce a succession of changes in the conductivity of the selenium corresponding in frequency to the musical vibrations within the limits of hearing; and upon further consideration it appeared to him that all the audible effects obtained from varieties of electricity could also be pro duced by variations of light acting on selenium. In an article in Science of Sept. 11, 1880, he says: "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 indefinitely 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 trans mitter and receiver. It was evidently necessary, in order to reduce this idea to practice, to devise an apparatus td be operated on by the voice of the speaker, by which variations could be produced in a parallel beam of light, corresponding to the variations in the air produced by the voice." But a difficulty was found in the fact that the resistance of selenium was too great to respond safIciently to the action of light; this, however, was overcome by reducing this resistance—from some half million ohms to 300 in the dark, and to 155 in the light. The fundamental features of the selenium photophone are best given in prof. Bell's own words: " We have devised about fifty forms of apparatus for varying a beam of light in the manner required. The best and simplest form consists of a plain m irror of flexible material—such as silvered mica or microscopic glass. Against the back of this mirror the speaker's voice is directed. The light reflected from this mirror is thus thrown into vibration corresponding to those of the cliaphrag,m itself. In arranging the apparatus for the purpose of reproducing sound at a distance any power ful source of light may be used, but we have experimented chiefly with sunlight. For this purpose a large beam is concentrated by means of a lens upon the diaphragm mirror, and miner reflection is again rendered parallel by means of another lens. The beam is received at a distant station upon a parabolic reflector, in time locus of which 13 placed a sensitive selenium cell, connected in a local circuit with a battery and telephone." The loudest effects obtained from light were produced by rapidly interrupting, the beam by a perforated rotating disk, revolving over the face of :mot iwr perforated disk, with holes corresponding. Audible mnsieal tunes were produced from the light of a candle.