PRODUCTION OF SOUND BY RADIANT ENERGY. 327 



DISTANCE FEOM FOCAL POIXT OF LENS AT ^VHICH SOUNDS BECOME INAUDIBLE 



WITH DIFFERENT SUBSTANCES. 



Metres. 



Zinc diaphragm (polished) 1'51 



Hard-rubber diaphragm 1*90 



Tin-foil " 2-00 



Telephone " (japanned iron) 2*15 



Zinc " (unpolished) 2'15 



White silk, (in receiver shown in Fig. 1.) 3-10 



White worsted, " " " 4-01 



Yellow worsted, " " " 4'06 



Yellow silk, " '' " 4-13 



White cotton-wool, " " " 4-38 



Green silk, " " " 4-52 



Blue worsted, '' " " 4-69 



Purple silk, " " " 4-82 



Brown silk, " " " 5-02 



Black silk, " " " 5-21 



Red silk, " " '^ 5-24 



Black worsted, " " " 6-50 



Lampblack. In receiver the limit of audibility could not be determined, on 



account of want of space. Sound perfectly audible at a distance of. .10*00 



Mr. Tainter was convinced from these experiments that this field 

 of research promised valuable results, and he at once debased an ap- 

 paratus for studying the effects, which he described to me upon my 

 return from Europe. The apparatus has since been constructed, and 

 I take great pleasure in showing it to you to-day. 



1. A beam of light is received by two similar lenses (A B, Fig. 

 10), wdiich bring the light to a focus on either side of the interrupting 

 disk (C). The two substances, whose sonorous powers are to be com- 

 pared, are placed in the receiving vessels (D E) (so arranged as to ex- 

 pose equal surfaces to the action of the beam) which communicate, by 

 flexible tubes (F G) of equal length, with the common hearing- tube 

 (H). The receivers (D E) are placed upon slides, which can be moved 

 along the graduated supports (I K). The beams of light passing 

 through the interrupting disk (C) ; are alternately cut off by the swing- 

 ing of a pendulum (L). Thus a musical tone is produced alternately 

 from the substance in D and from that in E. One of the receivers is 

 kept at a constant point upon its scale, and the other receiver is moved 

 toward or from the focus of its beam until the ear decides that the 

 sounds produced from D and E are of equal intensity. The relative 

 positions of the receivers are then noted. 



2. Another method of investigation is based upon the production 

 of an interference of sound, and the apparatus employed is shown in 

 Fig. 11. The interrupter consists of a tuning-fork (A, Fig. 11, ), 

 w^hich is kept in continuous vibration by means of an electro-mag- 

 net (B). 



A powerful beam of light is brought to a focus between the prongs 



