376 
POPULAR SCIENCE REVIEW. 
Selenium and the Photophone forms the subject of a recent communica- 
tion, from Mr. Graham Bell and Mr. Sumner Tainter, to the American 
Association. If the facts therein recorded bear the test of further examina- 
tion, a very important and startling addition has been made to our know- 
ledge of the possible relations between sound and light. The discovery is 
best given in the author’s own words, as reported by our weekly contem- 
porary, Engineering. 
1 The final result of our researches/ say the atithors, ‘ has evidenced the 
class of substances sensitive to light vibrations, until we can propound the 
fact of such sensitiveness being a general property of all matter. We have 
found this property in gold, silver, platinum, iron, steel, brass, copper, zinc, 
lead, antimony, German silver, Jenkin’s metal, Babbitt’s metal, ivory, cellu- 
loid, gutta-percha, hard rubber, soft vulcanized rubber, paper, parchment, 
wood, mica, and silvered glass; and the only substances from which we 
have not obtained results are carbon and thin microscopic glass. We 
find that when a vibratory beam of fight falls upon these substances they 
emit sounds — the pitch of which depends upon the frequency of the 
vibratory change in the fight. We find further that, when we con- 
trol the form or character of the fight-vibration on selenium, and 
probably on the other substances, we control the quality of the sound 
and obtain all varieties of articulate speech. We can thus, without a 
conducting wire, as in electric telephony, speak from station to station, 
wherever we can project a beam of fight. We have not had opportunity 
of testing the limit to which this photophonic influence can be extended, 
but we have spoken to and from points 213 metres apart; and there 
seems no reason to doubt that the results will be obtained at whatever dis- 
tance a beam of light can be flashed from one observatory to another. The 
necessary privacy of our experiments hitherto has alone prevented any 
attempts at determining the extreme distance at which this new method of 
vocal communication will be available.’ 
They then proceed to describe the peculiar properties of selenium under 
the influence of fight, remarking that 1 all observations by previous authors 
had been made by means of galvanometers ; but it occurred to us that the 
telephone, from its extreme sensitiveness to electrical influences, might be 
substituted with advantage. Upon consideration of the subject, however, 
we saw that the experiments could not be conducted in the ordinary way for 
the following reason : The law of audibility of the telephone is precisely 
analogous to the law of electric induction. No effect is produced during 
the passage of a continuous and steady current. It is only at the mo- 
ment of change from a stronger to a weaker state, or vice versa , that any 
audible effect is proposed, and the amount of effect is exactly proportional 
to the amount of variation in the current. It was, therefore, evident that 
the telephone could only respond to the effect produced in selenium at the 
moment of change from light to darkness, or vice versa , and that it would be 
advisable to intermit the light with great rapidity, so as to produce a succes- 
sion of changes in the conductivity of the selenium, corresponding in fre- 
quency to musical vibrations within the limits of the sense of hearing. For 
we had often noticed that currents of electricity, so feeble as to produce 
