SCIENCE. 
133 
a sound of definite quality through the atmosphere. The curve 
that would graphically represent the changes of light would be 
similar in shape to that representing the movement of the air. I 
do not know whether this conception had been clearly realized by 
“ J. F. W.," of Kew, or by Mr. Sargent, of Philadelphia; but to 
Mr. David Brown, of London, is undoubtedly due the honor of 
having distinctly and independently formulated the conception, 
and of having devised apparatus — though of a crude nature — for 
carrying it into execution. It is greatly due to the genius and per- 
severance of my friend, Mr. Sumner Tainter, of Watertown, 
Mass., that the problem of producing and reproducing sound by 
the agency of light has at last been successfully solved. 
The first point to which we devoted our attention was the reduc- 
tion of the resistance of crystalline selenium within manageable 
limits. The resistance of selenium cells employed by former ex- 
perimenters was measured in millions of ohms, and we do not 
know of any record of a selenium cell measuring less than 250,000 
ohms in the dark. We have succeeded in producing sensitive 
selenium cells measuring only 300 ohms in the dark, and 155 ohms in 
the light. All former experimenters seemed to have used platinum 
for the conducting part of their selenium cells, excepting Werner 
Siemens, who found that iron and copper might be employed. 
We have also discovered that brass, although chemically acted 
upon by selenium, forms an excellent and convenient material ; 
indeed, we are inclined to believe that the chemical action be- 
tween the brass and selenium has contributed to the low resistance 
ot our cells by forming an intimate bond of union between the 
selenium and brass. We have observed that melted selenium be- 
haves to the other substances as water to a greasy surface, and we 
are inclined to think that when selenium is used in connection 
with metals not chemically acted upon by it, the points of contact 
between selenium and the metal offer a considerable amount of 
resistance to the passage of a galvanic current. By using brass 
we have been enabled to construct a large number of selenium 
cells of different forms. The mode of applying the selenium is as 
follows : The cell is heated, and, when hot enough, a stick of 
selenium is rubbed over the surface. In order to acquire con- 
ductivity and sensitiveness, the selenium must next undergo a 
process of annealing. 
W e simply heat the selenium over a gas stove and observe its 
appearance. When the selenium attains a certain temperature, 
the beautiful reflecting surface becomes dimmed. A cloudiness 
gradually extends over it, somewhat like the film of moisture pro- 
duced by breathing upon a mirror. This appearance gradually 
increases, and the whole surface is soon seen to be in the metallic, 
granular or crystalline condition. The cell may then be taken off 
the stove, and cooled in any suitable way. When the heating 
process is carried too far, the crystalline selenium is seen to melt. 
Our best results have been obtained by heating the selenium 
until it crystalizes, and continuing the heating until signs of melt- 
ing appear, when the gas is immediately put out. The por.ions 
that had melted instantly re-crystallize, and the selenium is found 
upon cooling to be a conductor, and to be sensitive to light. The 
whole operation occupies only a few minutes. This method has not 
only the advantage of being expeditious, but it proves that many 
ot the accepted theories on this subject are fallacious. Our new 
method shows that fusion is unnecessary, that conductivity and 
sensitiveness can be produced without long heating and slow cool- 
ing; and that crystalization takes place during the heating pro- 
cess. We have found that on removing the source of heat imme- 
diately on the appearance of the cloudiness, distinct and separate 
crystals can be observed under the microscope, which appear like 
leaden snow-flakes on a ground of ruby red. Upon removing the 
heat, when crystalization is further advanced, we perceive under 
the microscope masses ol these crystals arranged like basaltic 
columns standing detached from one another, and at a still higher 
point ofheating the distinct columns are no longer traceable, but 
the whole mass resembles metallic pudding-stone, with here and 
there a separate snow-flake, like a fossil, on the surface. Selenium 
crystals formed during slow cooling after fusion present an en- 
tirely different appearance, showing distinct facets. 
We have devised about fifty forms of apparatus for varying a 
beam ol light in the manner required, but only a few typical vari- 
eties need be shown. The source of light may be controlled, or a 
steady beam may be modified at any point in its path. The beam 
may be controlled in many ways. For instance, it may be polar- 
ized, and then affected by electrical or magnetic influences in the 
manner discovered by Faraday and Dr. Ker. The beam of polar- 
ized light, instead of being passed through a liquid, may be re- 
flected from the polished pole of an .■lectro-magnet. Another 
method of affecting a beam ol light is to pass it through a lens of 
vaiiable focus. I observe that a lens of this kind has been invent- 
ed in France by Dr. Cusco, and is fully described in a recent paper 
in “ La Nature; ” but Mr. Tainter and I have used such a lens in 
our experiments for months past. The best and simplest form of 
apparatus for producing the effect remains to be described. This 
consists of a plain mirror of flexible material — such as silvered 
mica or microscopic glass. Against the back of this mirror the 
speaker's voice is directed. Trie light reflected from this mirror is 
thus thrown into vibration corresponding to those of the diaphragm 
itself. 
In arranging the apparatus for the purpose of reproducing sound 
at a distance, any powerful source of light may be used, but we 
have experimented chiefly with sunlight. For this purpose a large 
beam is concentrated by means of a lens upon the diaphragm mir- 
ror, and, after reflection, is again rendered parallel by means of 
another lens. The beam is received at a distant station upon a par- 
abolic reflector, in the focus of which is placed a sensitive selenium 
cell, connected in a local circuit with a battery and telephone. 
A large number of trials of this apparatus have been made with 
the transmitting and receiving instruments so far apart that sounds 
could not be heard directly through the air. In illustration, I 
shall describe one of the most recent of these experiments. Mr. 
Tainter operated the transmitting instrument, which was placed 
on the top of the Franklin schoolhouse in Washington, and the 
sensitive receiver was arranged in one of the windows of my labora- 
tory, 1325 L street, at a distance of 213 metres. Upon placing the 
telephone to my ear I heard distinctly from the illuminated re- 
ceiver the words: “Mr. Bell, if you hear what I say, come to the 
window and wave your hat." In laboratory experiments the 
transmitting and receiving instruments are necessarily within ear- 
shot of one another, and we have therefore been accustomed to 
pooling the electric circuit connected with the selenium receiver, 
so as to place the telephones in another room. By such experi- 
ments we have found that articulate speech can be reproduced by 
the oxy-hydrogen light, and even by the light of a kerosene lamp. 
The loudest effects obtained from light are produced by rapidly in- 
terrupting the beam by the perforated disk. The great advantage of 
this form of apparatus for experimental work is the noiselessness 
of its rotation, admitting the close approach of the receiver without 
interfering with the audibility of the effect heard irom the latter ; 
for it will be understood that musical tones are emitted from the 
receiver when no sound is made at the transmitter. A silent mo- 
tion thus produces a sound. In this way musical tones have been 
heard even from the light of a candle. When distant effects are 
sought another apparatus is used. By placing an opaque screen 
near the rotating disk the beam can be entirely cut off by a slight 
motion of the hand, and musical signals, like the dots and dashes 
of the Morse telegraph code, can thus be produced at the distant 
receiving station. 
We have made experiments, with the object of ascertaining the 
nature of the rays that affect selenium. For this purpose we have 
placed in the path of an intermittent beam various absorbing sub- 
stances. Professor Cross has been kind enough to give me his as- 
sistance in conducting these experiments. When a solution of 
alum, or bisulphide of carbon, is employed the loudness of 
the sound produced by the intermittent beam is very slightly 
diminished ; but a solution of iodine in bisulphide of carbon 
cuts off most, but not all, ot the audible effect. Even an ap- 
parently opaque sheet of hard rubber does not entirely do this. 
When the sheet of hard rubber was held near the disk interrupter 
the rotation of the disk interrupted what was then an invisible 
beam, which passed over a space of about twelve feet before it 
reached the lens which finally concentrated it upon the selenium 
cell. A faint but perfectly perceptible musical tone was heard from 
the telephone connected with the selenium. This could be inter- 
rupted at Will by placing the hand in the path of the invisible 
beam. It would be premature, without further experiments, to 
speculate too much concerning the nature of these invisible rays ; 
but it is difficult to believe that they can be bent rays, as the effect 
is produced through two sheets of hard rubber containing between 
them a saturated solution of alum. 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. 
It is a well-known fact that the molecular disturbance produced 
in a mass of iron by the magnetizing influence of an intermittent 
electrical current can be observed as sound by placing the ear in 
close contact with the iron. It occurred to us that the molecular 
disturbance produced in crystalline selenium by the action of an 
intermittent beam of light should be audible in a similar manner 
without the aid of a telephone or battery. Many experiments 
were made to verify this theory without definite results. The 
anomalous behavior of the hard rubber screen suggested the 
thought of listening to it also. This experiment was tried with 
extraordinary success. I held the sheet in close contact with my 
ear, while a beam of intermittent light was focussed upon it by a 
lens. A distinct musical note was immediately heard. We found 
the effect intensified by arranging the sheet of hard rubber as a 
diaphragm, and listening through a hearing-tube. We then tried 
crystalline selenium in the form of a thin disk, and obtained a 
similar but less intense effect. The other substances which I 
enumerated at the beginning of my address were now successively 
tried in the form of thin disks, and sounds were obtained from all 
but carbon and thin glass. We found hard rubber to produce 
a louder sound than any other substance we tried, excepting an- 
timony, and paper and mica to produce the weakest sound. On 
the whole, we feel warranted in announcing as our conclusion that 
souuds can be produced by the action op a variable light from substances 
of all kinds, when m the form of thin diaphragms. We nave heard 
lrom interrupted sunlight very perceptible musical tunes through 
tubes of ordinary vulcanized rubber, of brass and of wood. These 
were all the materials at hand in tubular form, and we have had 
no opportunity since of extending the observations to other sub- 
stances. 
