Mercadier, as a general term signifying an apparatus for the production of sound by any form of radiant energy, limiting the words thermophone 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 obtained 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 achromatic 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 spectrum.
The disk-interrupter (F) was then turned with 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 obtained 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 maximum sound lay very far out in the ultra-red. Beyond this point the sound began to decrease, 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-