PHYsrcs. 269 



duction of sound by lij^lit. The apparatus devised for the purpose lie 

 calls a pbotophoue; and its action is based ou the fact that when an in- 

 termittent beam of light is made to fall on a disk of almost any mate- 

 rial, this disk emits sounds whose pitch is the; same as the number of 

 nitermissions. The substance most sensitive, however, is selenium, 

 whi(;h, as is well known, diminishes in its electrical conductivity by the 

 action of light. When therefore an electric current was passed through 

 a selenium cell and a telephone, the intermittent beam of light falling 

 on the selenium varied its resistance so as to reproduce an intermittent 

 electrical current of the same pitch distinctly audible in the telephone. 

 The cell was made of disks of brass, alternating with slightly smaller 

 disks of mica, the metal disks being alternately united electrically. In 

 the grooves between these metal disks melted selenium was placed, and 

 the whole was heated to such a temj^erature that the selenium passed 

 into the crystalline state. This cell, placed in the focus of a parabolic 

 mirror, constituted the receiver. The transmitter consisted simply of a 

 sheet of mica or thin glass, silvered, against the back of which the voice 

 could be directed by means of a mouth-piece. Ui)on this abeam of con- 

 densed sunlight was allowed to fall, whence it was reflected to the dis- 

 tant receiver. Ou talking into the mouth-piece, the beam of light varied 

 correspondingly, and the spoken words became audible in the tele- 

 l^hone. — (Am. J. Sei., HI, xx, 305, October, 1880. See also An)i. Chim. 

 Fhys., V, xxi, 399, 5(30, 1880 5 Nature, xxii, 500; xxiii, 10, 58, 1880.) 



Mercadier has studied the simi)ler forms of the photophone, which he 

 calls radiophone, a sort of optical siren, in which a rotating disk i)ierced 

 with holes is interposed in the i^ath of light-rays, causing intermittences 

 of a i)eriod corresponding to the speed. The disk used is simply a sheet 

 of black paper gummed on a glass disk to avoid the whistling sound 

 noticed with perforated disks. The receiving disks were tixed in a suit- 

 able holder, at the end of a short hearing-tube of India rubber. When 

 the substance used was opaque, like zinc, copper, etc., the loudness is 

 the same, whether polished or not. Thin disks are better than thick 

 ones, and glass and quartz gave good results. A film of smoke or i)aint 

 or of metallic silver on the front of the disk diminishes the ett'ect, while 

 blackening the back of the disk makes it louder. — {Nature, xxiii, 209, 

 December, 1880.) 



2. Dispersion and Color. 



J. W. Draper has described a new form of spectrometer, intended for 

 comparing the brilliancy of lights. It depends on the optical principle 

 that a light is invisible in presence of another sixty-four times stronger. 

 To produce it, the scale-tube is removed from the common three-tubed 

 spectroscope, and a piece of glass ground on both sides is placed against 

 the aperture. By x)lacing a gas-flame in front of this glass, the held 

 appears uniformly' illuminated. This is the extinguishing light. With- 

 out it, the spectrum of a lumhious flame placed before the slit appears 



