June 9, 1899.] 



SCIENCE. 



809 



Accordingly the frequency of the lens fork 

 was made slightly diiierent from that of the 

 source, bj^ loading it sufliciently to obtain 

 slow beats. Thus the phases of the one over- 

 took those of the other very slowly, and con- 

 sequeutl}' the interference bauds were ob- 

 tained, sloping first to the right aud then 

 after an interval to the left, the changes 

 occurring periodically and following each 

 other as slowlj' as desired. By means of a 

 suitable eye-piece with divided circle the 

 angle of this slope can be measured, and 

 gives immediately a means of measurement 

 of relative intensities. 



CAMERA. 



For some work the stroboscopic method 

 of direct observation was replaced by a 

 photographic method by which permanent 

 records of sound disturbances were ob- 

 tained aud intensities determined. In this 

 case the telescope with vibrating eye-piece 

 was replaced by a fixed lens system which 

 focussed a narrow band of fringes iipon a 

 sensitive film mounted upon a uniformly re- 

 volving cylinder, in a manner similar to 

 that employed by Raps.* The cylinder was 

 driven by a small motor, whose speed was 

 kept constant by Lebedew'sj method. Since 

 this photographic record can be made 

 equally well in case of irregular disturb- 

 ances of the air, the instrument, with the 

 receiving resonator removed from the sensi- 

 tive plate, affords an unequalled means of 

 studying the physical characters of a great 

 variety of sounds and noises, such as vowel 

 sounds and consonants, the notes of various 

 musical instruments, the calls of birds, the 

 cries of animals, bells, whistles, the din of 

 the streets, the rumble of thunder, etc. The 

 effect of the peculiar note of the sensitive 

 plate may be eliminated by means of dif- 

 ferential measui-es with plates of different 

 natural periods. 



*Wied. Ann., 1893, p. 194. 

 t Wied. Ann., Band 59, s. 118. 



SOURCE OF TONE. 



For the determination of the instrumental 

 constants, aud for fundamental researches 

 in sound, it is essential that the source of 

 sound be pure in tone, constant in inten- 

 sity, and that its intensity be easily varied 

 within considerable limits. It should also 

 be portable. The following arrangement 

 meets these requirements in a very satis- 

 factory manner. A tuning fork of about 

 the same pitch as the note desired was 

 driven by an electromagnet with a current 

 interrupted by a similar control fork, elec- 

 trically driven by the usual method of 

 self-interruption. The first fork was fast- 

 ened vertically upon a heavy iron base, aud 

 one of its tines was connected to a circular, 

 thin iron plate, of approximately, the same 

 pitch as the fork, by means of a short stiff" 

 wire. This plate formed a side of a Helm- 

 holtz resonator, constructed to give the note 

 desired aud rigidly supported. The motion of 

 the fork tine was in the direction of the wire, 

 i. e., perpendicular to the plane of the plate, 

 so that the vibrations of the fork were con^- 

 municated to the plate ; thus the air within 

 the source resonator is thrown into forced 

 vibrations of verj' nearly its own frequency. 

 Accordingly a very small vibration of the 

 fork causes the resonator to emit a very 

 loud tone. Its intensity depends upon the 

 current driving the source fork, and this 

 current is governed by a sliding resistance 

 and shunt. A single storage cell suffices 

 for the loudest tones, and J^ ampere pro- 

 duced a tone loud enough to be heard very 

 distinctly all over a large lecture room 

 (about 14x24 yards). This source was en- 

 closed in a heavy, padded box, so that only 

 the lip of the resonator protruded. Per- 

 fect silence could be obtained by simply 

 corking the mouth of the resonator with a 

 rubber stopper, so that a single and defin- 

 itely located source was obtained, and one 

 which is portable. 



Care was taken that no sound should 



