PHYsrcs. 269 



duction of sound by lij:>lit. The apparatus devised for the purpose he 

 calls a photoplione; aud its action is based on 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 

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

 which, 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 aud 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 ele(;trically. In 

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

 the whole was heated to such a temi)erature that the selenium jxissed 

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

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

 sheet of mica or thin glass, silvered, against the back of *7hich the voice 

 could be directed by means of a mouthpiece. Upon this a beam of con- 

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

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

 correspondingly, and the si)oken words became audible in the tele- 

 phone. — (Am. J. iSvL, HI, XX, 'M)3, October, 1880. See also Ann. Ghim. 

 Phys., Y, xxi, oDO, 5G0, 1880; Nature, xxii, 500; xxiii, 1(), 58, 1880.) 



Mercadier has studied the simpler forms of tlie photo])hone, which he 

 calls radiophone, a sort of optical siren, in which a rotating dislc pierced 

 with holes is interposed in the path of light-rays, causing interraittences 

 of a period corres])onding to the speed. The disk used is simply a sheet 

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

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

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

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

 the same, whether ])olished 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 effect, while 

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

 December, 1880.) 



2. Dis2)ersion 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 placing a gas-flame in front of this glass, the field 

 apiiears uniformly illuminated. This is the extinguishing light. With- 

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



