158 BULLETIN OF THE 



(3.) Another mode is as follows : The loudness of a musical tone 

 produced by the action of light is compared with the # loudness of a 

 tone of similar pitch produced by electrical means. A rheostat 

 introduced into the circuit enables us to measure the amount of 

 resistance required to render the electrical sound equal in intensity 

 to the other. 



(4.) If the tuning-fork (A) in Fig. 11 is thrown into vibration 

 by an undulatory instead of an intermittent current passed through 

 the electro-magnet, (B,) it is probable that a musical tone, electri- 

 cally produced in the receiver (F) by the action of the same current, 

 would be found capable of extinguishing the effect puoduced in the 

 receiver (G) by the action of the undulatory beam of light, in 

 which case it should be possible to establish an acoustic balance 

 between the effects produced by light and electricity by introducing 

 sufficient resistance into the electric circuit. 



Upon the Nature of the Rays that Produce Sonorous Effects in 

 Different Substances. 



In my paper read before the American Association last August 

 and in the present paper I have used the word " light" in its usual 

 rather than its scientific sense, and I have not hitherto attempted to 

 discriminate the effects produced by the different constituents of 

 ordinary light, the thermal, luminous, and actinic rays. I find, 

 however, that the adoption of the word " photophone" by Mr. Tain- 

 ter and myself has led to the assumption that we belived the audible 

 effects discovered by us to be due entirely to the action of luminous 

 rays. The meaning we have uniformly attached to the words 

 " photophone" and " light" will be obvious from the following pas- 

 sage, quoted from my Boston paper : 



" Although effects are produced as above shown by forms of 

 radiant energy, which are invisible, we have named the apparatus 

 for the production and reproduction of sound in this way the 

 ' photophone' because an ordinary beam of light contains the rays 

 which are operative." 



To avoid in future any misunderstanding upon this point we have 

 decided to adopt the term " radiophone," proposed by Mr. Mercadier, 

 as a general term signifying an apparatus for the production of 

 sound by any form of radiant energy, limiting the words thermo- 

 phone, photophone, and actinophone to apparatus for the production 

 of sound by thermal, luminous, or actinic rays respectively. 



