330 



THE POPULAR SCIENCE MOXTHLY. 



Mercadier, as a general term signifying an apparatus for the production 

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

 ])hone^ photophone., and actinophone to apparatus for the production 

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



M. Mercadier, in the course of his researches 

 in radiophony, passed an intermittent beam 

 from an electric lamp through a prism, and 

 then examined the audible effects produced in 

 different parts of the spectrum (" Comptes 

 Rendus," December 6, 1880). 



We have repeated this experiment, using 

 the sun as our source of radiation, and have ob- 

 tained results somewhat different from those 

 noted by M. Mercadier. 



A beam of sunlight was reflected from a 

 heliostat (A, Fig. 12) through an achromatic 

 lens (B), so as to form an image of the sun 

 upon the slit (C). 



The beam then passed through another ach- 

 romatic lens (D), and through a bisulphide-of- 

 carbon prism (E), forming a spectrum of great 

 intensity, which, when focused upon a screen, 

 was found to be sufficiently pure to show the 

 principal absorption lines of the solar spec- 

 trum. 



The disk-interrupter (F) was then turned 

 Avith sufficient rapidity to produce from five 

 to six hundred interruptions of the light per 

 second, and the spectrum was explored with 

 the receiver (G), which was so arranged that 

 the lampblack surface exposed was limited by 

 a slit, as shown. 



Under these circumstances sounds were ob- 

 tained in every part of the visible spectrum, 

 excepting the extreme half of the violet, as 

 well as in the ultra-red. A continuous increase 

 in the loudness of the sound was observed upon 

 moving the receiver (G) gradually from the 

 violet into the ultra-red. The point of maxi- 

 mum sound lay very far out in the ultra-red. 

 Beyond this point the sound began to de- 

 crease, and then stopped so suddenly that a 

 very slight motion of the receiver (G) made 

 all the difference between almost maximum 

 sound and complete silence. 

 2. The lampblacked wire gauze was then removed, and the in- 





